Original subtitles

NARRATOR: What do we really know about the planet we live on?

This giant spinning ball of rock.

The truth is,

something extraordinary is going on deep inside the Earth.

Powerful forces, mysterious processes

are happening thousands of miles beneath our feet.

And without them, life on our planet would be impossible.

The secret to life on Earth lies inside.

[Rumbling]

To discover how and why, we need to crack the Earth open...

...and travel all the way to the core.

A century ago,

Jules Verne's book "Journey to the Center of the Earth"

captured the world's imagination.

Of course, in reality, it's an impossible journey.

In the center of the Earth,

there are titanic pressures and extreme temperatures.

They make 99% of the planet beneath us

inaccessible to humans.

It is easier to design something to descend into the sun

than it is to design something

to go to the center of the Earth,

because the temperatures are as high or higher

than the surface of the sun,

but the pressures are unimaginably large.

NARRATOR: Because scientists can't travel to the core

and see for themselves,

they have to work out other ways to understand it.

It's not easy studying something

you'll never be able to see or touch.

LATHROP: We can see hurricanes coming.

We can see fronts coming that will have violent thunderstorms.

All of that predictive power

comes because we can observe the atmosphere.

We don't have anything like that in the interior of the Earth

because we don't have any detailed measurements

of what's happening in the core.

We don't really know any of the motions in the core.

We don't know how the temperatures are varying.

We don't know what storms are brewing down there.

NARRATOR: But Lathrop is determined to find out,

so he's building his very own planet Earth

at the University of Maryland.

So we've been seven years in construction of this experiment.

Built to try to match as many parameters as possible

with the Earth's core.

It's a model of both the outer and inner cores of the Earth.

NARRATOR: It might look like a crazy experiment,

but investigating the Earth's interior

is more than just scientific curiosity.

Life on Earth's surface, where we live,

actually depends on processes taking place

deep inside our planet.

If we can figure them out,

then we'll be closer to understanding

how and why life exists and what its future could be.

LATHROP: And the hope is,

by building a laboratory model of a planetary core,

or the Earth's core,

that we can probe in detail what's happening

and work toward getting a predictive science,

being able to predict what's going to happen

toward the future for the Earth's core.

NARRATOR: Lathrop is not alone.

Around the world, scientists are probing the planet

in every way possible

to solve the mysteries of the deep Earth.

They're studying volcanoes...

...measuring vibrations from earthquakes

to perform seismic X-rays of the planet...

...building complex laboratory models...

...and discovering that the world beneath our feet

is stranger and more fantastic than they could ever imagine.

It's full of incredible riches,

monumental structures, and bizarre creatures.

They've found there's actually more life beneath the surface

than above it...

...and more water than in all of the oceans.

Down here, there are even raging storms

more violent than the planet's worst hurricanes.

And somehow this mysterious world deep inside the planet

shapes our own.

But to discover how is a huge challenge.

LATHROP: Almost any basic quantity that you imagine

might be changing down there.

There's a whole host of interesting questions

that you'd like to know about the core

but that you can't unless you go there.

NARRATOR: There are many mysteries in the deep core

but perhaps none so powerful as gravity.

Gravity keeps the moon and thousands of man-made satellites

in their orbits.

And even out here

it prevents molecules of gas from floating off into space.

This immense force

comes from the massive dense interior of our planet.

The closer we get to Earth, the stronger this force becomes.

By 62 miles up, gravity has collected enough gas

to form a cocoon around the Earth.

This is the Earth's atmosphere.

It protects us from meteorites, absorbs lethal radiation,

and insulates the Earth from the freezing temperatures of space.

And what's most important...

It gives us the air that we breathe.

It's simple. No gravity... no atmosphere.

No atmosphere... no life.

There's another force of nature inside Earth

that's just as vital to life.

We take it for granted that life gets its energy from the sun.

True, its nuclear furnace does warm our atmosphere,

drive our weather, and make our food grow.

Without the sun, life on Earth would quickly disappear.

But forces from deep inside the Earth

played a vital role in creating life in the first place.

Life survives today because of a careful balance

between the energy of the sun on the outside

and the energy coming from inside Earth's core.

The most visible sign

of the seething energy inside our planet are volcanoes.

They erupt through cracks in the crust,

the planet's fragile outer shell.

This layer is only 30 miles thick.

All of the Earth's volcanoes

release just a tiny fraction of the energy

locked beneath the surface.

The Earth's inner energy is so powerful,

it can thrust rock layers high in the air,

creating whole mountain ranges

such as the Guadalupe Mountains in New Mexico.

These layers were once a flat seabed

until the Earth's heat pushed them 8,000 feet into the sky.

In this churning, heaving action,

cracks and fissures let in water,

which dissolves the soft limestone rocks

below the surface.

Here in New Mexico are the magnificent Carlsbad Caverns.

One chamber is so large,

it could comfortably accommodate a jumbo jet.

For Peter Scholle,

these caverns are a geological treasure trove.

We're 850 feet below the surface of the Earth here

in the lower cave of Carlsbad Caverns.

We are amongst a bunch

of limestone stalactites and stalagmites.

This cave has probably a couple of miles of passage.

There are other caves

that have literally hundreds of miles of passage.

In many cases, there are actually rivers

that flow through them for tens or even hundreds of miles.

NARRATOR: The eerie stalactites growing downward

and the stalagmites growing upward

were deposited by the water over thousands of years.

[Rumbling]

Our journey from the surface to the core

reveals more spectacular surprises

as we head further downward.

Just below the surface, it's cold, dark, seemingly dead.

Then, very quickly, everything changes.

As we go even deeper, it gets warmer, then hot.

The next stop on our journey...

a mysterious cave below the Mexican desert.

This is what the Earth's inner energy can do.

At nearly 40 feet long,

these are the largest known crystals in the world.

They're what's left of an underground lake

rich in minerals.

The lake was turned into a boiling cauldron

by red-hot magma erupting from below.

As the hot water percolated through the crust,

these giant crystals

grew from the minerals dissolved in the water.

Today, the chamber is still a scorching 120 degrees...

so hot, scientists can only work 30 minutes at a time,

even in their climate-controlled suits.

LATHROP: But the deep interior is quite unsuitable for people.

Pressures are high, temperatures are high.

And early on, people going to mines

realize it gets hotter as you go deeper.

And so there's this fascination then

with this inhospitable interior

to what is otherwise a pleasant surface we live on.

NARRATOR: But the energy inside the Earth

can do more than make mountains and hollow out caves.

In the 1960s,

scientists discovered it can move entire continents.

The Earth's crust is formed

from seven massive sections called plates.

What researchers realized

is that these plates were all shifting.

In some places, they're pulling apart,

in others, smashing together.

Mountains are the crumple zones of these collisions,

and some are truly spectacular.

These are the Swiss Alps,

where two continents crashed together.

High peaks, like the Matterhorn,

testify to the immense scale of the forces unleashed.

It's literally a piece of Africa sitting on top of Europe.

Every year, these mountains grow by a quarter inch.

The Earth is always in motion.

Our mountains and continents slide around the Earth's surface

driven by energy from deep inside the planet.

But as this driving force reshapes the surface,

it reshapes life as well.

It can change and transform the course of life.

The evidence is here...

1.5 miles down inside a vast coal seam.

700 miles long and 120 miles wide.

212 million tons of coal.

All the coal on Earth

is the fossilized remains of a superforest

that once dominated the surface of our planet.

360 million years ago, there was an explosion of life on Earth.

It was more diverse,

more abundant than it's ever been since.

And it was all because of the way

that forces inside planet Earth had shaped the surface.

Go back in time.

That driving energy at the heart of the planet

had pushed the continents together

into a single giant landmass wrapped around the equator.

On this supercontinent, known as Pangaea,

there were vast lowland swamps and tropical rainforests.

It was a massive hothouse

and led to the creation of millions of new species.

This period of time is known as the Carboniferous era.

The closest scientists can get

to those conditions on Earth millions of years ago is here...

the Okefenokee nature reserve in southern Georgia.

Dr. Fred Rich is exploring how the inner Earth and life

are interconnected.

DR. RICH: There were large landmasses at the equator.

So you have to imagine this flat landscape just above sea level,

very well-watered, in the tropics.

And that paleogeography and the weather conditions,

the meteorology that followed from that,

led to the appearance of forests

that were unlike anything that had ever existed on the planet.

NARRATOR: It wasn't just that the forests were big.

The trees were monsters, too.

DR. RICH: Huge plants...

Some of these are reckoned to have been 70 to 100 feet high

and perhaps as much as 5, 6 feet in diameter...

...lived across this immense moist landscape.

And plants grew until they got so big or so old

that they simply fell over.

NARRATOR: These huge trees and dense forests

had a profound effect on the atmosphere.

They sucked up carbon dioxide and pumped out oxygen.

DR. RICH: High humidity.

Tremendous amount of oxygen exchange.

I mean, these plants were photosynthesizing.

So, understandably, these were oxygen pumps.

And they were similarly pulling huge amounts of CO2

out of the air.

NARRATOR: 360 million years ago,

the proportion of oxygen in the air

was 60% greater than it is today.

The high levels of oxygen

led to another dramatic effect on the Earth's creatures.

It supersized them.

There were poisonous centipedes 6 feet long.

2-foot cockroaches.

Even dragonflies the size of sea gulls.

DR. RICH: Dragonflies that we find in this swamp are large,

and they're certainly numerous.

But the dragonflies of the Carboniferous

would have been much bigger.

Easily three, four times the size,

based on what we have for fossil evidence.

NARRATOR: Instead of alligators,

the dominant predators were giant toads.

Alligators would have been replaced by large amphibians...

amphibians probably as large as the alligators

that we have in these modern swamps

but looking differently, perhaps.

NARRATOR: New species that changed the evolution of life,

all because the energy inside our planet reshaped its surface.

This strange lost world existed long before humans,

but its story was sealed into the Earth's rocks in coal.

The forest first became peat.

This was then squeezed under tons of rock,

where it started to dry out.

Now, in the process of this brown messy sediment

becoming coal,

the first thing we would need to do is get rid of the water.

Earthly processes do that simply by loading the sediment.

So the longer the sediment is in the ground,

the longer it has been buried, subjected to geothermal heat

that's coming from the interior of the Earth,

the more the sediment is compacted,

and the more the water is driven out.

NARRATOR: So the Earth's internal energy

had reshaped the landmass to make life possible,

then broke it apart and buried the remains deeper and deeper

until the heat and pressure

transformed the ancient forests into coal...

fossilized remains of a lost era.

As we go deeper on our journey,

there are other riches for humans to exploit.

2.5 miles down,

we pass a glittering seam of gold being formed.

Boiling fluids full of dissolved gold

bubble up through the cracks in the rock.

The higher it rises, the cooler it gets,

until the dissolved gold finally settles into seams.

Earth's thin crust...

home to life in all its complex, colorful, infinite variety.

Below it is an inhospitable, lifeless world.

Or so it seemed.

Scientists are now finding life deep inside Earth.

It's a remarkable discovery made in the world's deepest mines.

This is the Witwatersrand region of South Africa.

The mines here reach 2.5 miles inside Earth's crust.

It seems like they stretch a long way down.

But in reality, they barely scratch the surface.

This is a hostile environment for a human being.

It's 130 degrees Fahrenheit, 100% humidity,

and extremely cramped.

The mines are so deep,

the miners have to descend in two stages.

A single elevator cable stretching 2.5 miles

would snap under the strain.

It's so far down, the journey can take two hours.

Like the miners,

these biologists from Bloemfontein University

risk heatstroke as they descend into one of the mines.

But they're not interested in gold.

They're looking for life...

colonies of extraordinary creatures

that thrive in these extreme conditions...

...bacteria they believe may be direct descendants

of the very first life-forms on Earth.

Leading the team is Professor Derek Litthauer.

You've got communities of bacteria.

And possibly even fungi.

We don't know yet.

But probably mostly bacteria living in there.

And the kind of populations you get in there

are usually determined

by the chemical composition of the water.

But our past experience

has been that there's some unique stuff in there.

NARRATOR: The scientists tap into ancient underground water

released during the mining process.

The water and the bacteria inside it

have remained undisturbed for billions of years.

These bacteria are tough.

All they need to survive is rock, water, and scorching heat.

LITTHAUER: There's an amazing diversity of life underground,

even in the deep subsurface.

In some areas, we can expect life

possibly even down to 10 kilometers below surface.

And they are extremely sophisticated,

very highly specialized for the environment in which they live

off the nutrients that they can get in the rocks.

NARRATOR: It's an extraordinary discovery

that has transformed biologists' understanding

of the origins of life.

The bacteria are the latest additions

to a strange group of creatures

that thrive in extreme conditions

called extremophiles.

[Bubbling]

In the 1960s,

astonished scientists found bacteria

living in Yellowstone's boiling acid pools.

Then in the 1970s,

biologists discovered life 1.5 miles down in the oceans

close to vents in the seafloor called black smokers.

These life-forms thrive on nothing more

than volcanic gases.

If life exists in such hostile conditions...

...it suggests a teeming mass of life

could exist beneath our feet.

It's been estimated that all the bacteria inside Earth

could weigh more than all the life aboveground put together.

It also raises an intriguing possibility...

that life may have started not on the surface

but deep within the Earth.

There's more diversity and more life in the deep subsurface

than we have above surface.

The implications for this,

in terms of the evolution of life in the universe,

are quite astounding,

because the old concept that life could have started

in very calm, serene, warm pools on the surface of the Earth...

That may be completely wrong.

Life may have started in the subsurface.

NARRATOR: If life began underground,

then somehow at some time in Earth's history,

it found a route to the surface.

Perhaps the Earth's inner energy,

as it pushed through the crust,

took the extremophiles to the top.

Or maybe it hitched a ride on a black smoker...

a kind of extremophile elevator to ground level.

Or floated up in thermal hot springs,

boiling up from deep in the Earth.

How far down primitive life could survive is uncertain.

But to explore what lies beyond the deepest mine

pushes technology to its limits.

The only way down this far is to drill.

But pressure and heat put a limit

on even our most sophisticated drill bits.

The deepest hole ever drilled bored just 7.5 miles

into the Earth's 30-mile crust.

In the 1970s, the Soviets race

to drill the world's deepest borehole in Russia.

The drill bit was so long,

it bent and stretched like a piece of elastic.

But even at this depth,

we are less than halfway through the Earth's surface layer,

the crust.

It's only 1/500th of our journey to the core.

7.5 miles is like traveling from downtown Chicago

into the suburbs.

But it's another 4,000 miles to the center of the Earth.

That's like commuting from Chicago to London.

Scientists may be restricted

to exploring the thin top layer of the Earth's crust,

but their journey of discovery isn't over.

They've found other ingenious ways of exploring inside Earth,

and in the process

discovered more surprising connections

with the evolution of life itself.

Life on Earth might have started deep in the Earth's crust,

but violent upheavals even further down

played a crucial role in pushing life

on to the next stage of evolution...

one that would lead to all life as we know it.

Remarkably, we know this from the ancient rocks

of the crust itself.

These mountains

in Western Australia's Karijini National Park

are made from rock that's 3.5 billion years old.

They used to be the bed of an ancient sea.

Their red color comes from iron ore

imbedded right in the rock.

But the iron is evidence of something remarkable...

...because they were formed

during one of the most important events

in the story of life on Earth.

The bands of red iron ore were once layers of sediments,

and they contain evidence of very primitive life-forms.

Martin Van Kranendonk is a geologist

who's spent a lifetime studying these rocks.

Each one of these little bands

is only about the length of a thumbnail,

and it was maybe deposited in a year.

So you can see here,

you've got hundreds of feet of deposited sediments.

It represents hundreds of thousands of years

of geological time.

NARRATOR: These were no ordinary sediments.

They contain fossils of rock structures called stromatolites,

created by some of the earliest living things...

simple bacteria.

It's hard to imagine,

but this immense volume of iron-rich rocks

was actually formed by tiny microscopic organisms

that formed structures such as preserved here

in this very old rock.

This is an example of a stromatolite

that's built by single-celled organisms in this rock,

which is 3.45 billion years old.

This is the oldest fossil on the planet.

NARRATOR: Incredibly, these bacteria

are still making these distinctive rock formations

just 400 miles to the west.

These strange-looking mounds are giant stromatolites

built by the bacteria.

VAN KRANENDONK: Well, stromatolites are rocks,

but they're rocks that are made by living microorganisms

or, as we call them, microbes.

And so these stromatolites actually grow

by precipitating rock.

So they build up layer by layer, but only very slowly.

NARRATOR: The bacteria also produced something else,

something which kick-started a biological revolution...

oxygen.

If life did start underground,

maybe it eventually found its way to the surface,

propelled upward by those forces within Earth.

And once they'd reached the surface,

those bacteria found a new way to harness energy,

not from the rocks and the heat of the deep Earth,

but from sunlight... the process we call photosynthesis.

And one of the most important by-products of photosynthesis

is oxygen.

These stromatolites are incredibly important for us.

They're really the precursors

to allow life to evolve from the oceans on to land

and to breathe air.

NARRATOR: Without oxygen,

complex life as we know it simply wouldn't exist.

But oxygen also changed the composition of the planet,

creating the iron ore in the crust.

At the time, most of the iron on the surface

was dissolved in the oceans, making them appear bright green.

But the newly released oxygen bonded with all the iron

to make iron oxide, or rust.

The iron oxide fell to the seafloor,

and the seas turned blue.

Eventually, the iron oxide formed the deposits

we see in the Karijini mountains.

Layer upon layer of iron oxide exists in the Earth's crust

thanks to primitive bacteria.

It's the ore from which we extract

1.7 billion tons of iron each year,

and it's also rich in oxygen.

In fact, there's 20 times more oxygen

locked up in the bands of iron ore

than there is floating in the atmosphere.

It's another example of how the world we know

has been shaped by the incredible forces

deep inside the planet.

But where do these forces come from?

We now enter the part of the Earth

that holds the answer... the mantle.

It's a dynamic mass of churning rock

kept moving by energy from the core...

the powerhouse of the planet.

Below the 30 miles of surface crust,

we now move deeper, further than any human has ventured,

into the Earth's mantle.

The mantle is the real key

to understanding how our world works.

When you see flowing lava,

it's easy to think that the mantle is liquid.

In fact, it's nearly 2,000 miles straight down

of hot but solid rock.

It makes up 80% of the Earth's volume.

Nothing can live here.

But what happens at these depths is vital to life on Earth.

The mantle may be beyond our reach,

but sometimes it reaches us.

The solid rock liquefies

when the massive pressure on the mantle is suddenly released

through fissures and cracks in the crust.

The radical change in pressure transforms the rock into lava.

The rock of the mantle beneath the Earth's crust

is inaccessible.

But against the odds,

there are some places where mantlerock

has been forced to the surface.

One of them is on the Lizard Peninsula

on the southernmost tip of England.

On this peaceful beach

is evidence of something violent and powerful...

a piece of mantlerock that broke away

and was forced upward 30 miles

by the churning movements of the crust.

For geologists like Robin Shail,

it's the perfect place to study mantlerocks,

which are normally way beyond his reach.

How do they compare with other rocks on the surface?

What do they tell us about what's inside planet Earth?

DR. SHAIL: The rocks here look completely different.

They have colors which vary from greens

through to oranges and yellows.

This is typical of mantlerocks

wherever they're exposed at the Earth's surface.

NARRATOR: Like no other rocks we know,

mantlerock is very hard and very heavy,

nearly twice the weight of granite.

It's a dense mass of minerals rich in heavy elements

such as iron and magnesium.

And it's the source of gemstones

such as the distinctive green peridot.

Close up, structures are revealed

that could only have been formed

under extreme temperature and pressure.

Here on the Earth's surface, this rock seems solid enough.

Deep underground, however, it becomes very different,

something that behaves more like fudge.

When we look at this mantle peridotite, it appears solid.

In contrast, when mantlerocks... or fudge... are warmer,

you can actually stretch and make it flow.

And the significance for this

is that these weak layers within the mantle

allow the overlying plates to move slowly across.

NARRATOR: A solid that flows may seem strange,

but the mobility of the mantle is vital to life on Earth.

Because currents of heat circulate upwards

from the core through the mantle,

the plates of the crust can move around on the surface.

Without this shifting geology, there'd be no continents,

and the conditions for life would never have existed.

DR. SHAIL: Without these zones in the mantle

that allow the plates to move across the Earth's surface,

we would basically have a geologically dead planet.

We would have no plate movement.

We would have no mountain ranges.

We would have no major ocean basins.

So the mantle is absolutely critical.

NARRATOR: These are the deepest rocks visible

on the Earth's surface.

To look further into the mantle,

scientists must find another way.

Inside the Earth's mantle,

crushed beneath 100 miles of rock,

the pressure is 50,000 times more

than we feel at the surface,

like carrying 20 Titanics on your shoulders.

It's in this hostile environment

that some of the Earth's greatest treasures are forged.

The pressure creates diamonds.

It crushes carbon into the hardest mineral

known to science.

But we don't have to dig 100 miles to find them.

Diamonds exist just a few hundred feet below the surface.

They were forced up through the crust

by violent prehistoric eruptions

triggered by the Earth's internal heat.

Today, miners excavate these extinct volcanic vents

in search of diamonds.

The Letseng diamond mine

is located in the mountain kingdom of Lesotho...

...a small country in the heart of South Africa.

The diamonds are imbedded in rock called kimberlite

inside an old volcanic pipe.

It's the job of company geologists

like Claire Palmer to find them.

DR. PALMER: We're standing within the pipe,

the original eruptive pipe that formed.

And the original earth surface would have been

at least 200 meters above our heads.

And we're actually, in the mining process,

reexcavating that pipe.

NARRATOR: Most of the diamonds on Earth

exploded through the surface during huge volcanic eruptions

one billion years ago.

DR. PALMER: These volcanoes erupted at supersonic speeds.

So you can imagine the power with which it explodes.

Similar to that of Mount St. Helens.

But Mount St. Helens' eruption

moved laterally across the Earth,

whereas these eruptions were actually a lot more vertical

in their expanse.

NARRATOR: These violent eruptions

exploded minerals from 100 miles down

upward to the surface in minutes.

Today, the diamonds are locked inside this volcanic rock.

There's only one way to get them out.

Letseng is a valuable mine.

All these diamonds were recovered

in just over two weeks.

These diamonds are known worldwide

for their very high quality

and yield the highest dollar per carat in the world.

The Letseng diamond mine

is famous for its very large diamonds.

One of our most famous is the Lesotho Promise...

603 carats, which was recovered in August 2006.

And it sold on tender for $12.4 million U.S.

NARRATOR: Not all diamonds are perfect.

Some have microscopic flaws.

A perfect diamond is worth a lot more money.

But for geologists, these flaws are the real treasures.

They're tiny fragments of primitive mantle

trapped inside the diamond,

and they're the deepest samples it's possible to capture.

They tell a remarkable story.

Like time capsules,

they hold the key to unlock secrets

of the Earth's very early history.

From their chemistry,

scientists can deduce that most of these diamonds

are 3.2 billion years old.

They can even figure out they were forged 100 miles down.

Diamond samples from different parts of the world

show large variation in their composition.

That suggests the mantle was a churning dynamic place,

even in the early history of the planet.

From below 100 miles,

very few rock samples reach us on the surface.

But this isn't the end of our journey to the core.

There is another way to see what's down there.

It's like an X-ray image of planet Earth.

Most of the time,

we're unaware of the power locked inside our planet.

But sometimes there are violent reminders.

[Rumbling]

Earthquakes are the result of processes

taking place deep in the interior.

Propelled by the slow movement of the mantle,

the great plates that make up the Earth's crust

constantly grind into, over, and under each other.

Pressure builds until something snaps.

When this happens, the Earth shakes, heaves, and rolls.

The results can be catastrophic,

especially when they happen in populated areas.

This earthquake in China in 2008

killed 70,000 people and cost $150 billion worth of damage.

Big earthquakes are disasters,

but they're also windows on the deep interior of the planet.

Scientists can make use of the shattering power of earthquakes

to help understand the Earth's most remote depths.

They use a worldwide network of devices called seismometers

to trace earthquake vibrations

as they travel through the planet.

The data produced can help fill in our picture

of the deep Earth.

Professor Ed Garnero uses this technique to study the mantle...

all 1,800 miles of it.

GARNERO: When an earthquake happens,

the waves travel away from the earthquake

through the planet in the interior and on the surface...

in the same way, when you drop a rock in a pond,

you see the rings getting bigger and bigger and bigger

from the drop zone.

So, what we do in seismology is,

we have these sensitive microphones all over the planet

that record the ground shaking.

And so we keep track of the precise time it gets here.

So when you use a bunch of these instruments in concert,

you can start to say something

about the material the waves travel through.

NARRATOR: Just as doctors use sound waves

to picture a baby in the womb,

the waves from earthquakes can tell scientists

about the world concealed deep beneath the Earth's crust.

The waves travel through and bounce off structures

within the planet.

GARNERO: So if you have enough seismic data,

you can start to characterize the shapes of things

inside the planet that are reflecting the seismic energy.

NARRATOR: And because earthquake waves

travel differently through different materials,

we know our planet is made of many layers,

like an onion.

The waves show the mantle extends downward for 1,800 miles

and offer the first glimpse of our ultimate destination...

the Earth's core.

Ed Garnero's results

show intense activity within the mantle.

They reveal how convection currents of hot solid rock

constantly circulate through the whole layer.

It's too slow to observe directly.

But speed it up and it's clear...

over millions of years... the mantle is in constant flux.

Resembling mushrooms,

the vertical columns in his animations

show the steady movements of the Earth's interior.

GARNERO: So, what we're looking at here

is a convection calculation depicting things...

When they get to the top, they cool off, and fall back in.

Just like a lava lamp, you know,

the blob goes up and then its heat goes away

and it falls back in.

So that's what's happening here...

the cycling of material in Earth's mantle

over millions of years.

And this is a process that's happening today.

NARRATOR: These convection currents through the mantle

transfer heat from the core to the crust...

heat that drives and pushes the continental plates

on Earth's surface.

In this way,

the roaring energy of the core shapes the world we live in.

The crust consists of two kinds of plates...

oceanic plates and continental plates.

Ocean plates are heavier, so when the two collide,

the oceanic plate plunges downwards

under the lighter continental plate.

Whole sheets of crustal plate

extend right down to the edge of the core.

GARNERO: As that plate descends

and drags some of the water down with it

and the water... some of the crust sediments

are still saturated... they make their way down.

That water can actually be stored in the mantlerock.

NARRATOR: Over millions of years, descending ocean plates

have dragged so much water into the mantle

that scientists estimate there's now more water

below the Earth's surface than above it.

GARNERO: Take all the water

from the oceans and lakes and glaciers...

everything on the surface of the Earth...

and anywhere between 2 and 10 or 12 amounts of that

can actually be stored in the Earth.

NARRATOR: If all this water rose to the surface,

there would be flooding on a biblical scale.

No land could survive.

Eventually, sea levels would rise

21/2 miles above the peak of Mount Everest.

Luckily for us, it will never happen.

But some of this underground water

does make its way back to the surface.

The water carried down by ocean plates into the mantle

become superheated and drives back toward the surface.

A change in pressure liquefies the hot mantlerock.

Mixed with expanding water,

the lava punches up through the crust,

where it erupts with spectacular force.

Mount St. Helens is the most famous American volcano

created at a plate boundary.

The pulverized rock and steam that billowed out of the volcano

following its 1980 eruption

was once part of the plate beneath the Pacific Ocean.

There's a ring of explosive volcanoes like Mount St. Helens

circling the Pacific Ocean.

It's called the Ring of Fire.

Each one marks the spot

where the Pacific plate dives into the mantle below.

We're now entering the lower mantle,

a region at the edge of scientific understanding.

Nobody knows what it looks like,

but scientists speculate the hostile conditions here

may create bizarre chemical effects.

GARNERO: If you were to be able to go into the mantle,

you would see exotic things,

chemical things that we're not quite

we fully understand right now, but there's evidence for it.

And you'd see a lot of different kinds of layering.

Just like when you're driving in your car

and you see a roadcut, you can see the layered rock.

NARRATOR: But in a few places, something disturbs these layers.

Plumes of hot mantlerock rise up from the core to the crust.

If you happen to live above one of these plumes,

the result can be both creative and destructive.

So you would see little isolated conduits...

the details of which we're not fully clear on,

but we think they could be 100 miles in diameter...

very hot material that works its way to the surface

and gives rise to these things that we call hot spot volcanoes.

You can see in this image,

you have hot plumes of material coming up to the surface.

And the stuff that comes out is what we see coming out

of places like Hawaii and Easter Island

and Kerguelen Islands and such.

And this animation was made with things called tracers...

these little black dots.

So you can get an appreciation for how slowly the material

moves across the core-mantle boundary

until it finds its little plume upwelling and then... foom...

they shoot up quite rapidly.

NARRATOR: Some of the world's largest volcanoes...

Yellowstone...

Iceland...

Hawaii...

sit right above these gigantic mantle plumes.

Hawaii's Big Island is evidence of their creative power.

Measured from the ocean floor,

this is the world's tallest single mountain...

4,000 feet higher than Mount Everest.

And every foot of it is made from lava

spewed out from the top of a mantle plume.

The surface plate is constantly moving,

while the mantle plume stays still,

so the magma keeps punching through the crust

in different places

and leaves a chain of extinct volcanic islands in its wake.

But while mantle plumes have the power

to create entire island chains,

they also have the power to destroy vast amounts of land.

Yellowstone's geysers and mud pools may delight tourists,

but they are signs that the park sits on top

of a vast mantle plume.

With a crater 45 miles long and 35 miles wide,

this is one of the world's largest supervolcanoes.

Geologist Hank Heasler wants to understand its behavior.

DR. HEASLER: There's been many destructive volcanic episodes

in Yellowstone... three massive eruptions...

one at 2. 1 million years ago,

which is one of the largest that we as geologists can define

on the face of the Earth,

one at 1.3 million years ago, and one at 640,000 years ago.

NARRATOR: Yellowstone may not look much like a volcano.

It's more of a wide depression.

But that's just because of its sheer size.

DR. HEASLER: Yellowstone is such a big volcano

that so much material has been erupted...

hundreds to thousands of cubic kilometers of magma

have been forcefully ejected into the air.

When all that magma is erupting,

the ground actually subsides into the void

created by the erupting magma.

NARRATOR: It's been 640,000 years

since Yellowstone last erupted.

Heat emissions from the park could be a sign

that the next eruption is overdue.

If the Yellowstone volcano does erupt,

it will unleash billions of tons of ash and gas

into our atmosphere.

It would block out the sun and plunge the world

into a devastating volcanic winter.

Mantle plumes are a key part

of the Earth's interior cooling system.

They have the power to create

some of the world's most beautiful

and dangerous landscapes.

The question is, what creates mantle plumes?

Nobody knows for sure.

But one thing is certain...

The answer lies somewhere in the boiling furnace

of the Earth's core.

1,800 miles down into the Earth,

just below us, is the core.

The Earth's outer core is a huge ball of liquid metal

bigger than the moon.

LATHROP: The conditions of the outer core

are really quite hostile.

Temperatures more than 3,000 degrees.

The pressure is just mind-boggling.

More than a million atmospheres of pressure.

If you could strip away the mantle

and just have the raw core,

it's quite hot and would be glowing intensely,

very much like the surface of the sun is glowing.

It's that hot.

NARRATOR: If we could open up a space

between the mantle and the core, this is what it might look like.

LATHROP: Just inside the mantle,

liquid metal meets the mantle.

There's probably, you know, a bit of a mushy zone,

where there's liquid metal mixing in

with the last bits of mantle material.

And then inside of that

is just this vast, deep ocean of liquid metal,

which is red-hot, flowing,

there's all this churning motion,

and probably things that are analogous to clouds,

in the sense of bits that are more dense and less dense

mixing about as the core convects.

NARRATOR: Seismologists can see what the outer core looks like

because seismic waves bounce off its liquid surface.

And scientists like Dan Lathrop

are discovering what's going on inside the core

by measuring the powerful electromagnetic energy

it produces... the Earth's magnetic field.

LATHROP: If you look at the pattern of magnetic field

on the outside of the Earth,

it's quite clear that that pattern is slowly moving

and slowing changing in a way that would be easily described

by it rising from a liquid metal

that's also slowly moving and slowly convecting.

NARRATOR: The Earth's magnetism

has been known about for more than 1,000 years.

And for centuries, explorers and sailors

have kept detailed records of our moving magnetic North Pole.

We now know that birds and animals use it to navigate

on their epic migrations across continents and oceans.

By the 1950s,

scientists understood that something made of metal

was responsible for the magnetic field.

It was the Earth's core.

Dan Lathrop wants to know how the field could be generated,

so he's built a model of the core,

a sphere filled with liquid metal.

Not iron, but sodium.

Iron would be too heavy and dangerously hot.

But sodium isn't perfect either.

Well, sodium has its pros and cons, without a doubt.

It's a very good electrical conductor...

an excellent electrical conductor...

so it gets us closer to being like a planet

in the laboratory experiments.

The cons are, it's a reactive liquid.

It is flammable, burns readily in air,

and also reacts violently with water.

NARRATOR: With the 13 tons of sodium safely sealed inside,

the 10-foot sphere starts to spin

to re-create the Earth's rotation.

Heaters keep the sodium molten.

Minutes later,

magnetic fields spill from the sphere in all directions.

Lathrop's experiment confirms

the way the Earth's magnetic field is generated.

Driven by the heat,

the convection currents in the core

combine with the Earth's rotation

to create a giant dynamo.

LATHROP: The dynamo is like an electrical generator,

but it's being driven by the motions

of the liquid outer core.

And that churning motion,

sort of turbulent convection in the core,

couples with the magnetic field

to continuously regenerate the magnetic field.

It's like the turning motion of the generator,

in this case then, is the churning of the convection.

NARRATOR: The magnetic field

is much more than a geological curiosity.

It's vital to life on Earth.

The field protects us from our closest, deadliest enemy...

the sun.

A giant nuclear reactor,

enormous storms rage on its surface.

These storms fling lethal radioactive particles

into space.

This is the solar wind, and Earth lies right in its path.

But like a stone in a stream,

the Earth's magnetic field parts the flow of radiation,

diverting it around the planet.

We sit in a protective pocket of magnetism...

the mystery of life

made possible by the mysterious core of the planet it inhabits.

The Earth's magnetic field is absolutely critical

for Earth to be a habitable planet,

in the sense that the quite violent radiation

coming from the sun stream around the outsides of a bubble

formed around the Earth by the magnetic field.

So the magnetic field extends a sort of shield,

the magnetosphere, which protects us and the atmosphere

from most of the radiation.

If that weren't there, the solar radiation

would be constantly bombarding the atmosphere,

actually eating away at the atmosphere,

and some of it then directly making it down to ground level.

NARRATOR: About 40,000 miles above the poles,

the charged solar particles

meet the outer reaches of the magnetic field.

Here, some are diverted down

toward the Earth's magnetic poles,

where they create spectacular auroras that glow in the sky.

These dazzling displays happen

when the particles slam into gas molecules

in the Earth's upper atmosphere.

Although beautiful,

these are a sign of a ferocious battle

between the Earth's core

and an invading stream of solar radiation.

Our magnetic field protects us from other dangers,

not just from the sun.

Lethal cosmic rays made of radioactive particles

permeate deep space.

Down on Earth, we're unaware of them.

But up in space, it's a different story.

On July 20, 1969,

Neil Armstrong was the first man to set foot on the moon.

It was one of humankind's greatest achievements.

But on their way to the moon,

Armstrong and co-pilot Buzz Aldrin

saw flashes of light inside the darkened Apollo 11 module.

Bizarrely, they even saw the flashes with their eyes shut.

When they returned to Earth, they reported what they saw.

NASA scientists were mystified.

Six years later, they came to believe these light flashes

were the result of high-energy cosmic rays

penetrating the spacecraft and the crew members' eyes.

Armstrong and Aldrin were exposed to these rays

because the Apollo craft

was near the edge of the safety shield

of the Earth's magnetic field.

MAN: 3, 2, 1.

And liftoff of Discovery.

NARRATOR: In the years since,

at least 39 astronauts

have developed some kind of eye cataract

a few years after exposure to this dangerous radiation.

Without the Earth's magnetic field,

we would all be exposed to these dangers.

And it's the core that is our great protector.

We know the magnetism comes from the rotation of the core

and the turbulence of the molten metal within it.

But how can we work out exactly what's going on inside the core?

Peter Olson is one scientist who's devised an experiment

that could offer an explanation.

Well, what we have here

is nothing more than a large tank of water on a turntable.

And what it's intending to simulate

is the Earth's outer core.

And we're going to inject some heavy dye

into this big tank of water,

and we're going to see the effects of the rotation

on the turbulence.

There's a turbulent plume

trying to sink to the bottom of the tank.

But it starts to feel the effect of the rotation,

and you can see it gets twisted up into kind of a helix.

And it's this helical type of flow in the Earth's core

that we think is so critical

for generating the Earth's magnetic field.

Ordinary turbulent motions

don't have this kind of helical structure to them.

But by virtue of the effect of the Earth's rotation,

the turbulence in the core is made helical.

NARRATOR: These helical columns

might explain the Earth's magnetic field.

They represent liquid-iron columns,

which could work like the wire coils inside an electromagnet.

As they move with the Earth's rotation, they create magnetism.

2,500 miles below the Earth's surface...

could there really be molten columns of liquid iron

hundreds of miles high?

OLSON: As a consequence of this turbulent motion

of the liquid iron,

electric currents are flowing in the core.

And the geomagnetic field that we see at the surface

is actually the result of these electric currents.

So there is no bar-magnet or permanent-magnet effect

of any significance inside the core of the Earth.

The magnetic field there is produced by electric currents.

NARRATOR: This delicate feedback system

makes the core seem extremely fragile.

Without heat or rotation, it wouldn't work.

To demonstrate,

Olson simply switches off the tank's rotation.

The water keeps moving, but as it slows down,

the convection currents gradually collapse.

If this happened in the core,

the Earth's magnetic shield would soon disappear.

Deep inside the Earth's core,

something mysterious is happening.

Swirling currents of molten metal

are creating a magnetic field that envelops the planet.

We depend on this field

to protect us from deadly solar radiation.

But scientific data

shows that magnetic field is weakening.

Over the past century,

the strength of the planet's magnetic field

has declined by nearly 10%, and scientists aren't sure why.

During most of mankind's history,

the magnetic field has been very strong.

And now it's weakening.

LATHROP: The Earth's magnetic field

has been studied for about 160 years.

And what people see is that the magnetic field

has slowly and steadily dropped in its strength.

NARRATOR: In one region,

the magnetic field is a third weaker.

It's here over the Atlantic Ocean,

just off the coast of Brazil.

It's known as the South Atlantic Anomaly.

This disruption in the magnetic field

stretches a quarter of the way around the globe,

and it's growing.

Every day in this area,

cosmic radiation reaches closer to the Earth's surface.

This protection that we get from the solar radiation

from the magnetic field is already weaker in that patch,

so it already has implications...

mostly for astronauts and people who run satellites.

OLSON: It's really come into prominence

since the advent of long-term orbiting spacecraft.

For example, the Hubble Space Telescope

has had enormous problems over the years

as it passes through the South Atlantic Anomaly.

NARRATOR: The problem is so bad

that when the billion-dollar Hubble Space Telescope

is above the area,

vital instruments are routinely shut down for protection.

[Radio chatter]

And near the core under the South Atlantic,

something even stranger is happening.

The magnetic field here hasn't just weakened,

it has totally reversed.

LATHROP: If you look at what

the magnetic field would be at the edge of the core,

the magnetic field down there

has already reversed in that patch.

Now, this could be a sign,

if this becomes deeper and broader,

that we're headed toward a reversal.

NARRATOR: A reversal is a total change in polarity

of the Earth's magnetic shield.

The North Pole flips to the south,

and the South moves north.

LATHROP: What a reversal is,

is when those North and South Poles reverse

so that you have a long, steady period

where they're in one orientation,

and then there's a reversal

and then a long, steady period in opposite reversal.

NARRATOR: Reversals have happened before.

We know this because, when lava cools,

it preserves evidence of the Earth's magnetic field.

Crystals inside the molten lava line up with the field.

When it solidifies,

it creates a record of its strength and direction

at that exact moment in time.

Studies of prehistoric lava flows

indicate that the last reversal happened 700,000 years ago,

when our apelike ancestors roamed the Earth.

You might think that, if the field is so stable

that it can persist for billions of years,

why should it suddenly decide to change?

But it does.

We know that the Earth's magnetic field

has reversed many hundreds of times.

What we don't know is when will it do it next?

NARRATOR: Neither do we know what will happen when it does.

The weakening magnetic field and the South Atlantic Anomaly

are the signs that we're about to experience the next reversal.

It could happen within the next 1,500 years.

OLSON: The rate of decrease is about 6% per century.

Now, that doesn't sound like very much, perhaps.

But in geologic terms, that's extremely rapid.

NARRATOR: No one knows what a reversal will mean

for life on Earth.

But while the magnetic field reverses,

we would lose its protection for several months.

Solar radiation would penetrate our electrical systems.

Surges would overload the world's power grids.

At the same time,

bats, birds, and whales could become disoriented

as their internal navigational systems are scrambled.

There could even be an increased incidence of cancer

as solar radiation attacks our cells' DNA.

We might see auroras appearing all over the planet.

Even over our major cities.

No one knows exactly when the next reversal will happen,

but the answer could lie even deeper inside the Earth

in the inner core.

It's the least understood, most remote,

and inaccessible place on the planet.

And somewhere in this hidden, hostile world

lies the key to the Earth's future.

The inner core is a rotating sphere of solid metal

floating inside the liquid outer core.

Billions of amps of electricity leap across its surface.

Hotter than the outer core,

the inner core's heat is the ultimate driving force

behind the Earth's magnetic shield.

OLSON: The pressures are so high

towards the center of the Earth

because of the overlying weight of so much material,

that despite the fact that it's hot,

the material is still solid.

NARRATOR: Seismic studies tell us something else

about the inner core...

slowly but surely, it's growing.

Every year, it expands by one millimeter

as the planet loses heat.

Nobody has ever seen this process with the naked eye.

But in the lab, scientists can use their imagination

to show something similar.

LATHROP: So as the Earth cools,

the inner core grows by iron crystallizing onto it.

We could imagine what that looks like

by looking at ice crystallizing onto this cool sphere.

A lot of people who think about the core

sit around and argue about, what's that surface like?

Is it rough? Is it smooth? Is it mushy?

What we know is that,

from the earthquakes passing through,

if it is rough,

the thickness of that is less than a mile or so.

But that still leaves lots of room

for mushy zones or cavernous pits

and little mini mountains.

We really have no idea what that surface looks like.

But if you look at any other surface on the Earth,

on other planets elsewhere in the solar system,

they're all rough.

Even the surface of the ocean is rough,

of course, moving about with the waves.

And so my expectation is

that things are quite rough and quite complicated.

NARRATOR: Exactly how rough and complicated

is open to debate.

Dan Lathrop believes the inner core's surface

is probably covered in a forest of metallic projections.

They're called dendrites.

LATHROP: There's most likely a sort of rough surface

of these iron crystals, perhaps dendrites poking out.

And the whole core itself

has a sort of crystalline order to it.

So while it's roughly spherical,

it has crystalline bits growing out from it,

continuously growing larger.

NARRATOR: As the core cools, the dendrites grow.

It's a sign that heat is constantly being transferred

from the inner to the outer core.

The Earth is slowly cooling, just from its origin.

And whenever you have something

which is hotter on the inside and colder on the outside,

it tends to get flows going, vortices.

You know, think of them sort of like

big, tumbling, cloudlike motions,

but it's in the liquid metal in the core.

NARRATOR: This heat transfer is fundamental to life on Earth.

It powers the outer core and the Earth's magnetic shield.

But it won't last forever.

With planet Earth losing heat every second, every day,

one thing is certain...

the inner core will keep growing and cooling.

In the distant future,

the whole core will freeze solid.

For life on Earth,

the consequences of that are unthinkable.

The inner core of planet Earth is a mysterious place,

hotter than the surface of the sun,

yet it's solid metal.

The core radiates incredible heat energy outward.

At the same time,

it crushes everything down around it

with intense gravity.

There's no way to see it or sample it.

How did it get there? Where did it come from?

There are clues.

The Earth shares its origins with the other rocky planets...

Mars, Venus, and Mercury.

In the beginning, just after the sun lit up,

before the planets existed,

great clouds of cosmic debris orbited the newly ignited star.

These early building blocks

crashed into each other with massive force.

The bigger the objects became,

the greater their gravitational pull,

until eventually whole planets formed.

LATHROP: When a planet forms,

it forms from a hodgepodge

of all sorts of different materials.

And so the heavier bits would tend to fall under gravity

and accumulate into the interior of the Earth.

We know that the bits of material

that made up all of the inner planets

had quite a bit of iron in them...

just raw, metallic iron.

And that would tend to sink down eventually

to form this massive core of the Earth.

NARRATOR: The solar system is now complete and stable,

but the process of formation, called accretion,

is not quite over.

The spare parts left over

from the creation of the solar system...

asteroids, comets, meteorites...

still orbit the sun and still crash into the Earth,

like the one that created this...

Meteor Crater in Arizona.

It was formed by an impact 50,000 years ago.

And for cosmochemist Meenakshi Wadhwa,

it offers a glimpse of the forces and the materials

that created the Earth's core.

WADHWA: So Meteor Crater that you see here

was created by the impact of an object

probably that was about 300, 400 feet across.

And this was an event that was a sudden, catastrophic event.

A lot of energy was released...

something like 20 megatons or so.

NARRATOR: Lmagine a planet growing

from billions of impacts like this one,

each one delivering iron, nickel,

and the other elements that make the world around us.

They also delivered an enormous amount of heat energy.

WADHWA: You can see that there were large blocks

that were ejected out from the crater,

and there were actually material

probably tossed out to hundreds of miles from the crater

as a result of the impact.

NARRATOR: The impact here was so powerful,

it vaporized the meteorite.

But a few fragments survived.

So this particular meteorite is...

It's called a Canyon Diablo meteorite,

and it's an iron-rich meteorite which was part of the impactor

that created Meteor Crater.

It's very difficult, of course,

to actually sample a piece of the Earth's core,

but these meteorites right here provide us a window

into looking at planetary interiors.

And you can actually learn something

about core-formation processes

by looking at iron-rich meteorites.

NARRATOR: Close up,

you can see the crystalline structure of the metal

that exists right at the heart of our planet,

a planet that's unique in the solar system.

But what makes Earth so special?

If the other rocky planets were made the same way,

how come they're so different today?

What happened to them

might shed light on the future of our own planet.

Scientists look to them for clues

that can tell them more about the fate of the Earth's core.

And the planet that interests them most is Mars.

It's our nearest neighbor.

Like Earth, water once flowed on its surface.

It had a thick atmosphere.

But that was billions of years ago.

Today, the planet is a frozen desert.

Most of its water and atmosphere have vanished.

And even though Mars has a metal core,

its magnetic field is tiny.

Are these conditions a coincidence?

Or is Mars a vision of Earth's future?

MAN:in NASA's Mars Global Surveyor.

NARRATOR: In 1996, NASA launched the Mars Global Surveyor.

Its mission... to unlock the secrets of the red planet.

MAN:as America begins its journey back to the red planet.

NARRATOR: But in the process,

it unlocked some of our own planet's secrets,

shedding new light on the very center of the Earth...

the inner core.

The Global Surveyor's data astonished scientists.

It showed Mars' magnetic field is very weak,

but Mars' crust is intensely magnetized.

The implications for our planet are immense.

Like Earth, Mars once had a powerful magnetic field.

But at some point, the Martian core cooled and froze,

and its magnetic field collapsed.

The question is, could it happen to our planet?

Mario Acuna was one of the scientists

who built the magnetic sensors that gathered the Mars data.

He used it to create a map of Mars' magnetized crust.

He discovered that in one area, there is no magnetism at all.

And it corresponds with a particular physical feature.

One of the things that we observe

is this very large hole in Mars,

if we want to call it a hole.

It's really the remnants of a gigantic impact

that took place very early in Mars' history.

NARRATOR: This hole is an enormous meteor crater.

It was clear that the rocks here,

unlike those in the rest of Mars' crust,

hadn't been magnetized.

So the crater must have formed

after Mars' core stopped working.

Scientists think the meteor impact here

released so much energy,

it liquefied the planet's crust at the point of impact.

Crystals in the cooling lava

would have recorded the surrounding magnetic field,

just like they do on Earth.

But in the gigantic crater on Mars,

the rocks bear no record of being magnetized.

Scientists theorize

that's because the magnetic field no longer existed

when the impact occurred.

The continent-sized crater was created 4 billion years ago.

It means the dynamo in Mars' core

stopped working when the planet was in its infancy.

DR. ACUNA: For the first time,

we could time when the dynamo disappeared.

And since Mars was formed only 41/2 billion years ago,

that means that the dynamo only lasted

a few hundred million years.

NARRATOR: The reason for Mars' premature death

lies in its size.

Mars is half the diameter of Earth,

so it cooled more quickly.

Its core froze,

and its magnetic shield collapsed.

The fate of life on Mars was sealed.

The planet lay exposed to the solar wind.

Its atmosphere and water eroded away.

DR. ACUNA: The fact that the magnetic field disappeared

had a tremendous effect on the loss of water by Mars.

We are looking for something like 1,500 feet of water

over the entire planet Mars to have disappeared from Mars.

NARRATOR: Earth is much larger than Mars,

so its core is still hot, still working.

But the lesson of Mars is unavoidable.

Eventually, Earth's own core will cool

until the convection columns inside the outer core collapse,

and then our magnetic shield will come down.

Without it, solar radiation will strip away

both our atmosphere and liquid water.

Then Earth will become a dead and desolate place.

But we don't need to panic just yet.

The extreme temperatures in the inner core

suggest we have plenty of time left,

perhaps even billions of years.

Nearly 4,000 miles from the surface,

we have reached our destination...

the very center of the Earth.

This is the hottest part of the planet.

Temperatures reach 12,000 degrees,

hotter than the surface of the sun.

And with no gravity, it's like nothing else on Earth.

The very center of the Earth is

probably the most un-Earthlike place on the planet,

in the sense that gravity gets weaker as you go down,

and when you hit the center, there's no gravity left.

There's no direction which means down.

Gravity is absent.

The temperature is the hottest spot on the Earth.

And so it's this sort of white-hot, gravityless,

very high-pressure... just crushing pressures

of all of the weight of the rest of the Earth

all pushing down on you.

So it's extremely inhospitable

and extremely strange at the same time.

NARRATOR: The world beneath our feet

may seem like an alien place,

but our journey has shown

it's very much part of life aboveground.

Everything about it is just right.

The Earth spins at precisely the right speed,

and it's exactly the right size

to allow some heat loss from the core, but not too much.

As a result, we have our magnetic field.

The mantle is just mobile enough

to allow currents of heat to move upward

so we have our continents to live on.

And our gravity is just the right strength

to bind our atmosphere and oceans to the surface.

From the crust to the core,

every layer, every rock, every piece fits together

to make life upon the surface possible.

The secret of all life as we know it

lies deep inside planet Earth.