Original subtitles

Male narrator: In the beginning, there was darkness,

and then, bang,

giving birth to an endless expanding existence

of time, space, and matter.

Every day, new discoveries are unlocking the mysterious,

the mind-blowing, the deadly secrets

of a place we call The Universe.

Throughout its more than 4 billion years of existence,

planet Earth has endured some extremely rough days.

But what were the very worst?

Was it a shattering encounter with a planet

that got too close?

- Had anything been there,

it definitely wouldn't have survived.

Narrator: Or a hot shower of deadly rays

that almost scoured the planet clean of life?

- The blast from the impact would have laid waste

a thousand miles in every direction.

It's literally the sound heard around the world.

Narrator: From runaway volcanism that led to vast extinctions

to tiny microbes that triggered a global deep freeze,

life has teetered perilously close to the brink

time and time again.

- More life died then than any time since

and maybe any time before.

This was the biggest and the baddest of them all.

Narrator: Are the darkest hours behind us

or lurking in the distant future?

Take cover as we count down

the seven worst days on planet Earth.

[dramatic music]

J J

You may think you've had some really rough days...

Stuck in gridlocked traffic,

dodging a sea of humanity on the way to work,

or trapped in a torrential downpour

or blizzard.

But they can't even begin to compare

to the seven worst days on planet Earth.

- Throughout its history,

Earth has experienced some really bad days,

days that have killed off life, created life,

restructured the planet as a whole.

All of this together has formed the Earth that we live on today.

But during those really bad days,

we certainly wouldn't want to be here.

- From our point of view,

Earth has been through some pretty rough days,

catastrophic impacts that have really changed

the face of the planet

and changed the makeup of the flora and fauna—

The animals and plants— on the planet.

Narrator: One of Earth's worst days

came early in its history,

when an uninvited guest barged into our neighborhood

and threatened to destroy the planet

before life had a chance to blossom.

Blasting in as the seventh-worst day on Earth...

About 4 1/2 billion years ago,

our solar system was a battle zone.

Giant boulders collided and coalesced

to form planets.

Infant Earth took hit after searing hit.

- If we could somehow magically transport ourselves back in time

to be on the early Earth, it'd be very interesting,

I think, to watch one of these large-impact events.

The surface of the Earth itself was probably heated

to a near molten state.

The atmosphere would be very dusty.

It would be very hot.

So it would be a very alien world to us,

one that we actually couldn't survive on

just standing like we are here today.

Narrator: Pounded by incoming rocks,

Earth slowly grew to nearly its present size,

developing a solid crust, a thick mantle, an iron core,

and a hot, stormy atmosphere.

Some speculate that life may have taken a tenuous hold,

but then.. disaster.

A neighboring planet named Theia

had been edging into Earth's orbital path.

The intrusion ended in the most disastrous way imaginable:

a collision of worlds.

- It was a very violent event.

Imagine something the mass of Mars,

about 10% the mass of the Earth,

whacking in from space and blowing all its material in.

Whatever was on the Earth at that time

was radically changed by this event.

- When a Mars-sized object whacked into the Earth,

that was a bad day on Earth.

Had anything been there,

it definitely wouldn't have survived.

Narrator: It took hours

for the full scope of the catastrophe to unfold.

As the cores of the two planets melded,

Earth's atmosphere was blasted into space,

its crust liquefied at over 2,000 degrees Fahrenheit,

and vast portions of both Earth's and Theia's mantles

were ejected into orbit.

- It's not at all like what you see in the movies.

You can sit down and have a cup of coffee,

and it's still going on!

These are—you know,

big objects take a long time to occur.

So these aren't the normal flashes that you think about,

of bullets hitting the wall.

Narrator: The plume of ejected material

created a vast orbital ring before coalescing

into a new companion for the Earth:

the newborn Moon.

- That fully fledged planet merged with the Earth.

The iron cores would have come together,

and the silicate mantles were left on the Earth

and also accreted into a disc that was orbiting the Earth.

This disc, most of it, came back and crashed back onto the Earth,

but then a portion of it re-formed

and, over time, has been pushed out

and has become the Moon.

Narrator: It took about 150 million years

for Earth to stabilize

and its molten crust to re-harden.

But although the Theia impact was a very bad day on Earth,

it was a very good day for its ability to sustain life.

- When the Moon formed,

it stabilized Earth's axis of rotation.

Otherwise, gravitational tugs from other planets

would have caused it to chaotically vary,

and that would have been bad for long-term climate stability.

- Would we even have life as we know it

if that impact had not taken place?

Without a moon, you could really have a case

where your tropics,

within hundreds of thousands of years,

could rotate and become the arctic or the Antarctic,

and you kill everything.

Narrator: With the impact over

and the Moon now providing orbital stability,

the stage seemed set for a quiet future.

But that was not to be.

- You form the planets. You form the Moon.

Everybody's happy.

And all of a sudden, something terrible happens.

Narrator: Earth was suddenly caught

in the crossfire once again.

Number six on our countdown of worst days...

Approximately 150 million years after the Theia impact,

the Earth's crust cooled.

Some speculate that early oceans and even life may have evolved.

But another cataclysm would wipe the slate clean.

In the far reaches of the solar system,

the orbits of the outer gas giants

began to fluctuate,

disrupting vast swarms of asteroids and comets.

Trillions broke loose from their orbits

and plunged inwards towards the rocky planets,

launching an era of fiery destruction

known as the Late Heavy Bombardment.

- We know asteroids the size of Texas hit the Earth.

Any impact or that big causes the entire ocean to go away.

We sterilized the Earth over and over and over,

so the inference we have

is that life evolved over and over and over

just to be snuffed out again by these large impacts.

Narrator: The relentless bombardment

gouged out craters thousands of miles wide,

and the hits just kept coming

for at least 200 million years.

Today billions of years of erosion

and shifting plate tectonics

have erased any trace of the cataclysm

from the Earth's surface.

But it's a different story

on the cratered face of the Moon.

- We can see very clearly

its record on the face of the Moon.

The lava-filled basins were formed

during the giant impact events

that occurred during the Late Heavy Bombardment.

And so we know that large objects hit the Earth

during this time.

Narrator: But this celestial rain of fire

may have had a silver lining, literally.

New evidence suggests the Late Heavy Bombardment

delivered precious metals to the surface of the Earth;

things like silver, gold, and platinum.

Earth's original store of these elements

had sunk into its molten iron core

as the planet slowly cooled.

But now, with our planet already solidified,

this new supply of materials remained near the surface,

able to be mined by future humans.

- Today's technologies depend on a lot of the heavy elements

that were delivered to the Earth through impacts.

So it may have been very difficult for the life,

if there were life on Earth, during the heavy bombardment.

But it was very good for the Earth's crust

to have all of these materials that we now depend on

delivered to the surface of Earth through impacts.

Narrator: The Late Heavy Bombardment

was Earth's true baptism by fire.

But now, as we reach number five in our countdown

of the planet's worst days,

Earth catches the worst cold in history,

a climactic crisis so chilling that life itself

was in danger of being frozen in its tracks.

Both the Theia impact and the Late Heavy Bombardment

turned Earth into an inferno.

But now, as we continue our countdown

of the seven worst days on Earth,

we skate forward in time to a catastrophe not of fire

but of all-encompassing ice.

Number five on our countdown:;

the frozen cataclysm known as snowball Earth.

- We had these bad days,

and a snowball Earth is much longer than a bad day.

It's a bad hundreds of thousands of years.

Snowball Earth is that the planet becomes a snowball.

It cools enough that the oceans freeze

down to 10 feet, 20, 50 feet thick.

Imagine the arctic sea ice we see now,

except the whole planet has that.

Narrator: During a typical ice age,

glaciers invade the temperate zones,

often reaching as far south as modern New York or Paris.

But during snowball Earth,

the glaciers just kept coming

until they had encircled the entire planet

and frozen the oceans to a depth of up to a mile.

As average temperatures plunged to minus 75 degrees Fahrenheit,

the equator grew as cold then as the South Pole is today.

And this global deep freeze

may have happened not once but twice,

the first episode beginning 2.4 billion years ago.

- During the snowball Earth phase,

the entire Earth has a climate

quite similar to modern-day Antarctica.

It's extremely cold, extremely dry, extremely windy,

extremely inhospitable to life.

Narrator: The snowball Earth hypothesis

helps explain the discovery of glacial deposits

in areas that were once at the equator.

It's theorized that as the glaciers

marched on from the polar regions,

they reflected sunlight back into space

and created a powerful feedback loop,

turning the thermostat relentlessly lower.

- If the polar regions get too large,

if the snow and highly reflective ice coatings

get too far down in latitude,

they reflect more light back into space,

the Earth gets cooler,

the ice goes closer and closer and closer to the equator.

So it's thought to be kind of a runaway process.

So this leaves a freeze-over of the Earth.

Narrator: But what caused the first snowball Earth?

Scientists think the culprit

actually may have been life itself.

- The gas that was most prevalent

when life first appeared on the Earth

wasn't oxygen; it was methane.

Creatures at that time, single-celled bacteria,

would have used methane

as part of their normal living processes.

- Methane is a phenomenal greenhouse gas.

So if you had a lot of methane in the atmosphere,

you had, likely, very warm temperatures.

Narrator: But 2.4 billion years ago,

a new group of underwater microbes

began absorbing energy from the Sun

in a process called photosynthesis.

By converting carbon dioxide and water into energy,

photosynthesis polluted the Earth

with a toxic new waste product:

oxygen,

vast amounts of it.

- Prior to 2.4 billion years ago,

oxygen was a poison.

And any life that was on Earth did not use oxygen.

It was considered a waste gas.

All of that life that didn't like oxygen

was exposed to this rich oxygen environment,

and it caused a great deal of stress.

- Methane creatures were happy to live

in this oxygen-free world.

Now, when oxygen finally does appear,

it was like the worst calamity in their history.

Narrator. As oxygen flooded the world,

it oxidized methane,

turning it into carbon dioxide.

The methane-consuming creatures died off

in a vast extinction event

called the great oxygen catastrophe.

And as Earth lost

its warming blanket of methane gas,

the planet froze over.

But if oxygen-producing life was the primary trigger

of the first snowball Earth,

that life now needed to find a warm place

to escape the disaster it had caused.

- The Earth at that time

was even more energetic tectonically

than it is now.

The formation of the Earth produced enormous heat.

That heat comes to the surface. It's slowing.

But back then, many more volcanoes.

Around every volcano, there would have been

this melted lake-like zone where life could have thrived.

Narrator: To show how volcanism

could have carved out cozy niches for life,

all you need is a blowtorch and a big ball of ice.

- We're here at Carving Ice,

an ice house owned by Roland Hernandez,

an amazing ice sculptor who's created for us

this hemisphere of solid ice

which represents snowball Earth.

We've placed it up here on this dolly

so that we can heat it from below

with an industrial blowtorch,

to represent the type of heating that we would experience

on snowball Earth

from volcanism deep beneath the surface.

- Now let me just light this up.

[torch hissing]

Al right.

Passing you the torch.

- Okay.

[torch hissing]

Narrator: Scientists suspect that the extreme heat pressure

generated by volcanism beneath the Earth's surface

would have created cracks in the ice

for life to take refuge.

- Okay, so you can see how the cracks have formed

all throughout the ice along the top here.

These are very similar to the types of cracks

that would form during a snowball Earth event.

It's in these cracks where you can get

the mixture of a little bit of liquid,

a little bit of heat, a little bit of chemistry—

In other words, the perfect ingredients

for generating an environment

where life can take root and even thrive.

Narrator: Volcanism wasn't just key

to life's survival during snowball Earth.

It also may have forced the glaciers

to finally retreat.

- So you've got all of these gases

that are coming out of the volcanoes—

Things like water vapor, carbon dioxide,

sulfur dioxide, for example—

Which are escaping into the atmosphere.

And over time, over tens of millions of years,

you're building up

these levels of greenhouse gases.

At some point,

it's going to get so warm in the atmosphere

that the ice begins to melt.

Narrator. Both episodes of snowball Earth—

The first 2.4 billion years ago

and the second 600 million years ago—

Placed profound stresses on early life.

But oddly, both were followed

by enormous flowerings of new species.

- We know from the sedimentary record

that just before animals bloomed into existence,

an enormous amount of phosphorous

came into the ocean.

There were big rocks of phosphorous.

They eroded.

As that erosion occurred,

phosphorous levels in the ocean rose.

All of a sudden, life is able to just bloom.

This may have really caused the diversification two times—

Of the microbial world at 2.3 billion years ago,

but more importantly to us,

it really probably helped the diversification of animals

at 600 million years ago.

Narrator: Although snowball Earth

produced some of the planet's harshest weather,

life not only managed to survive;

it bounced back stronger than ever,

which is why this disaster

only ranks as number five on our list of worst days.

So far, on our countdown

of the seven worst days on Earth,

we've seen our planet battered

during the moon-forming impact,

baked by the Late Heavy Bombardment,

and almost completely frozen over by snowball Earth.

Now we travel forward over 100 million years

as the world is rocked

by one of the greatest mass killings in its history.

Number four on our list...

In the billion of years of life on Earth,

99% of all species that ever lived

have ultimately vanished,

mostly due to five mass-extinction events.

The first was during the Ordovician period

450 million years ago.

- The mass extinction during the Ordovician

is the most mysterious of the so-called big five,

simply because it was the one that was the oldest.

Every one after that was closer to us in time,

and the further back in time you go,

the more fuzzy our scientific view of the past is.

Narrator: During the Ordovician period,

the globe was covered with three major oceans

and four super continents.

The land was still barren,

but the oceans teemed with life,

including brilliant corals and the planet's first fish.

Then a mysterious cataclysm swept the seas,

and 60% of life on Earth vanished.

- There's a couple of possibilities.

Number one: it got cold again.

So why would it have gotten cold?

Well, it could have been that plate tectonics

moved all of the formerly warm continents

down to the polar regions,

and that dropped the temperature of everything.

Narrator: For years,

scientists blamed the Ordovician extinction

on a devastating ice age.

But now some researchers propose that the mass killer

could have been something from the cosmos,

like a gamma ray burst,

the biggest blast in the universe.

- A gamma ray burst is a colossal explosion of a star

that occurs asymmetrically.

There are two oppositely directed beams

of very energetic high-speed particles

and radiation that go zipping through space,

kind of like a laser beam.

If that beam is aimed right at Earth,

it can cause significant damage here.

Narrator: A large burst would vaporize

1/3 of the planet's protective ozone layer,

and creatures on the side of the Earth

facing the onslaught

would suffer lethal radiation exposure.

The destructive energy of a gamma ray burst

can be demonstrated here on Earth.

- Here's our globe representing the Earth

at the end of the Ordovician period,

a time when one group of scientists thinks

that a gamma ray burst

may have triggered a mass extinction.

The paper represents the Earth's ozone layer,

and the gamma ray burst

is represented by this propane torch.

[gas hisses]

[click]

Eight, nine, ten seconds' worth.

That's all they say it would take

to destroy about 1/3 of the Earth's ozone.

So of course, the gamma ray burst

didn't actually torch the Earth,

but it did destroy the ozone that it encountered.

And it would have messed with the chemistry

of the ozone molecules on this side of the globe.

And when the ozone was destroyed,

UV radiation could penetrate to the surface.

And the ultraviolet radiation is very lethal to life.

It's very destructive to DNA.

So it may be that a gamma ray burst

destroying the ozone layer

contributed to, or maybe even triggered,

a mass extinction at the end of the Ordovician.

Narrator: In addition to depleting the ozone layer,

a large gamma ray burst also could have ripped apart

nitrogen and oxygen molecules in the atmosphere,

producing a gaseous smog of nitrogen dioxide

that blocked sunlight

and set off a secondary disaster:

an ice age.

- It's not clear whether that could have been caused

by a gamma ray burst.

But at least part of the Ordovician extinction

seems to have been marked

by a huge decrease in the number of surface-dwelling creatures,

like on the Earth's surface

or in the surface layers of the ocean.

That may well indicate that it was ultraviolet radiation

that killed them off rather than some other factor.

Narrator: A gamma ray burst could be one explanation

for the first mass annihilation of life on Earth.

But a startling new hypothesis

points to a different cosmic force:

a bizarre and potentially deadly phenomenon

known as a bow shock.

- A bow shock forms when there's compression of material

in front of a moving object.

So for example, if you have a boat zooming along,

it creates a wave,

a compression in front of it

that then bends around as the boat moves through.

Well, our galaxy is moving rapidly through space,

so it can compress intergalactic gas

in front of it, heating it up,

creating cosmic rays.

And some of those cosmic rays can then hit the Earth.

Narrator: Our solar system oscillates

through the Milky Way,

traveling above and below the main disk of the galaxy

every 64 million years.

Once outside the galaxy's protective magnetic field,

our planet is vulnerable to deadly cosmic rays

generated by the galactic bow shock...

much like a wake boarder on a lake.

- Wake boarding would be a reasonable example

of a bow shock in space.

Hit it!

While the wake boarder is well within the confines

of the bow wake of the boat,

the water is very calm and smooth,

and he has no trouble standing up.

But during those periods when he moves

out toward the edge of the wake,

the water becomes a bit more turbulent

and a bit more chaotic,

and he has trouble standing up.

The same thing is true in space.

For most of the time,

the Earth and Sun are well within

the protective confines

of the Milky Way galaxy's magnetic field,

protecting it from the cosmic rays

and other radiation that's coming in from the bow shock.

But during those periods

when the Earth and the Sun rise above the galactic plane,

it's much more vulnerable

to all of these cosmic rays coming from the bow shock.

Narrator: Fossil records indicate

that the biodiversity of many species

may increase and decrease in 62-million-year cycles,

those cycles closely coinciding

with the times of greatest exposure

to cosmic rays from galactic bow shock,

which occur every 64 million years.

Could the two events be connected?

- Two of the mass extinctions

happened to have occurred at about the time

when the Sun was highest above the plane of our galaxy.

That suggests that there's something

about that particular location that's special.

Causing these mass extinctions.

Narrator: During the Ordovician extinction,

Earth could have been blasted with enough deadly radiation

from the galactic bow shock

to unleash genetic mutations and DNA damage

that wiped out over half of all species.

- This is not yet a well-tested idea.

It's just out there on the drawing board.

But it could explain

some of the possible periodicity in Earth's mass extinctions.

Narrator: But while the Ordovician extinction

was fatal to most species,

some grasped at the opportunity.

- It's like pulling weeds out of a garden.

You sort of clean out and weed out and thin out.

That seems to have happened

in the Ordovician mass extinctions.

Some creatures which died out allowed others to diversify.

Narrator: The Ordovician was the first mass extinction

but hardly the worst.

Number three on our countdown of the worst days on Earth:

it's the most famous die-off in prehistory.

But if you think you know what killed the dinosaurs,

you may be in for a surprise,

because some scientists

now argue that the common explanation

of a single giant impact on a single disastrous day

is itself about to be blasted away.

In our countdown to the very worst day on Earth,

no catastrophe has been the subject

of more public fascination and sensational headlines

than the bombardment

that ultimately cleared the path for mankind.

But today controversial new hypotheses

challenge the concept that a single disastrous impact

wiped out the dinosaurs

and led to the third-worst day on Earth...

- 65 million years ago,

we would have seen a world something like Africa

in terms of the number of animals out there.

But instead of big mammals, we have big dinosaurs,

herds of triceratops, herds of duckbill dinosaurs,

small to big predators, the largest being T. Rex.

[dinosaur roaring]

Narrator: Dinosaurs ruled the prehistoric world

for almost 200 million years.

Then something took out 2/3 of all living creatures,

including the giant beasts.

After years of searching for clues,

most experts now agree

that a single cataclysm doomed the dinosaurs.

- The evidence for an impact of a six-mile-wide asteroid

65 million years ago

is about as good as anything gets in science.

Narrator: 65 million years ago,

a rock the size of Mount Everest barreled down from the sky

and slammed into the Yucatan peninsula,

near modern Chicxulub, Mexico.

The impact unleashed

an unstoppable chain of destruction,

first locally then across the globe.

- The k-t is an asteroid-driven extinction.

A big rock from space came down,

clunked a few dinosaurs on the head,

but the kill mechanism

was really the secondary effects of this.

There would have been darkening of the atmosphere,

acid rain, a change in the entire biosphere.

Narrator. Proponents of this single-impact theory

point to a convincing array of evidence,

including high levels of the asteroid element iridium

at the k-t event's geological boundary,

or death layer,

and, of course, the smoking gun itself,

the underwater crater in modern Mexico.

But is this cold case finally closed?

Some scientists now argue

the Yucatan asteroid had accomplices.

- There seem to be various causes

that can trigger these,

and in some cases, several causes come together

in sort of a confluence of bad circumstances

to provide a particularly damaging extinction event.

Narrator: Millions of years before the k-t extinction,

vast lava flows had been pouring out of the Deccan traps,

a series of lava beds now located in southwest India.

These flood basalts once spewed out enough magma

to cover a million square miles

and belched up a toxic brew of gases

that could have altered the climate around the world.

- That period of eruption is pretty coincident

with the extinction 65 million years ago as well.

And in fact, they were going on for millions of years

before the k-t impact and before the extinctions

and so perhaps could be a complicating factor.

Narrator: In this scenario,

the dinosaurs received a double knockout blow...

First the rampant volcanism of the Deccan traps

then the Yucatan impact.

And a small group of dissenters goes even further,

claiming that multiple asteroids struck Earth

at roughly the same time 65 million years ago.

According to them, the Yucatan impact

wasn't the largest, only the best documented.

- Some people think

that there may have been multiple impacts,

a shower of impacts, for example.

Perhaps they came from a single object that broke apart.

Narrator: Supporters of this hypothesis

point to a suspicious geologic feature,

a 370-mile basin off the coast of India

more than three times larger than the Yucatan impact site.

But controversy swirls around this so-called Shiva crater—

Named after the Hindu god of destruction—

Including whether it's even a crater at all.

- While this feature certainly looks circular

and looks crater like,

there's no other evidence

that it's an actual impact structure.

Narrator. For some,

the dinosaur die-off debate rages on.

But most scientists agree the apocalyptic event

was a very fortunate day for small mammals

that were no longer dinner for T. Rex.

- Had it not been for the k-t impact

and the volcanism that was also

at the time of the extinction of the dinosaurs,

the mammals probably wouldn't have risen

to the prominence that they did.

And thank heaven for the rise of mammals,

because that led to the rise of humans.

Narrator: The k-t extinction raises a sobering question.

Could humans one day go the way of the dinosaurs?

That's what viewer Tom Moore from Adrian, Michigan,

wanted to...

- Well, Tom, Earth could definitely be hit

by another k-t sized comet or asteroid.

They're out there.

The good news is, we're tracking the positions

of most of the big ones.

So if we ever find one with Earth's name written on it,

hopefully, we'll be able to deflect it

before it reaches Earth.

Narrator: The k-t event toppled T. Rex,

but it only ranks as number three

on our countdown of worst days on Earth.

Number two was the mother of all extinctions,

the greatest die-off

that life on Earth has ever suffered.

It's also been

one of the biggest unsolved mysteries in science...

until now.

As we near the climax of our countdown,

we reopen one of the oldest cold cases,

an unsolved murder mystery

that happened 250 million years ago,

almost 200 million years before the downfall of the dinosaurs.

The second-worst day witnessed the greatest mass extinction

in the planet's history,

a cataclysm known as the Great Dying.

During the Permian period,

Earth was a wondrous world of mammal-like reptiles

and exotic sea creatures.

Then 90% of all life disappeared from the planet.

- This was a bad few hundred thousand years

on planet Earth.

But it was certainly the most catastrophic.

More life died then than any time since

and maybe any time before.

This was the biggest and the baddest of them all.

Narrator: For decades,

scientists have sifted through clues,

struggling to solve the mystery of the Great Dying,

when the survival of life itself seemed to hang by a thread.

One of the prime suspects:

a super volcano that tore Siberia open

for hundreds of miles,

erupted for at least a million years,

and produced the greatest lava flows ever known,

enough to bury the entire United States

thousands of feet deep.

- This wasn't a single giant volcano.

It was simply cracks in the Earth,

and you're, like, pumping out lava.

It's like a fire hose spewing lava everywhere.

Well, that certainly would have killed anything in its way.

- It's believed that you had these beds of coal

in what is now Siberia,

which then have lava erupting underneath them,

burning the coal,

putting all sorts of noxious greenhouse and other toxic gases

into the atmosphere,

and really putting strong stress onto the biosphere.

Narrator: The increased global warming

was fatal to most animals.

But plants usually thrive on carbon dioxide.

So why did they also succumb

at rates that have never been equaled?

- There's a second killer in the story

that we think— and just now are discovering—

And that's the presence for the first time on the planet

of massive amounts of hydrogen sulfide gas,

the same stuff you see with rotten eggs.

That's a true poison.

And it, with the heat, combined, I think,

to cause this greatest of mass extinctions.

Narrator: If the scientific jury is still debating

the cause of the Permian-Triassic extinction,

no one disputes that those were the darkest days

for life on our planet... so far.

But life on Earth cannot and will not survive

an event that scientists predict

is as unavoidable as death itself,

the number one worst day on planet Earth...

In 5 billion years,

our sun will reach the end of its life.

As it exhausts its nuclear fuel,

it will undergo a disastrous transformation,

bloating into a red giant star

200 times its original size.

As it swells, it will first engulf Mercury

then Venus.

The incinerated Earth

will either be pushed to a higher orbit

or swallowed itself.

The good news:

humans won't be around

the day our sun incinerates Earth.

The bad news: we will have been roasted long before.

- The Sun gets brighter about 10% every billion years.

And so in less than a billion years,

the Sun will be bright enough

that it's predicted that it will pump basically all—

Well, most of the carbon dioxide out of the atmosphere,

and so things that rely on carbon dioxide,

like plants, are going to have a hard time

surviving in a natural world.

Narrator: As plants shrivel and oceans evaporate,

biologists predict a future mega mass extinction.

The age of animals will end in about 500 million years.

- We have as much time in the future

to be alive as an animal as we've had in the past.

We're halfway through.

That's a sobering thought.

Narrator: We've seen Earth

smacked by the giant moon-forming impact,

strafed and sterilized during the Late Heavy Bombardment,

then frozen into a giant snowball.

A cosmic force ravaged an ancient water world

during the first mass extinction,

followed by a near total extinction

during the Great Dying

and the spectacular fall of the dinosaurs.

What else lies in store

before our planet is ultimately baked by the Sun?

- Earth will certainly continue to experience disasters

all the way up until the very end,

when the Sun's red giant phase brings an end to the story.

Narrator: History has proven

that Earth is an unpredictable planet,

orbiting in a very dangerous galactic neighborhood.

To survive, mankind must learn from the worst days of our past

and try to prepare for whatever the cosmos might throw at us.

Only by increasing our knowledge of the universe

can we hope to thrive and fully enjoy

our final 500 million years of life on Earth.