Original subtitles

>> Earth, a 4.5-billion-year-

old planet, still evolving. As

continents shift and clash,

volcanoes erupt, and glaciers

grow and recede, the Earth's

crust is carved in numerous

and fascinating ways, leaving

a trail of geological

mysteries behind. In this

episode, the Marianas Trench,

the deepest point on Earth, is

explored. Its sheer walls cut

seven miles into the Pacific

Ocean. The mystery of what

created this deep, dark chasm

takes science detectives on

some of the most dangerous

dives ever attempted, deep

into the abyss. Scouring the

ocean floor, scientists

uncover a strange,

undersea world of fiery

mountains, bizarre mud

volcanoes and the largest

geological structure on Earth.

Discoveries from this unique

underwater world will

revolutionize our

understanding of the powerful

forces that shape not just the

trench, but the Earth itself.

S01x02 The Deepest Place on Earth Original Air Date on February 17, 2009

Hidden deep beneath the waves

of the western Pacific lies

the Marianas Trench, the

deepest point of all the

oceans.

The first step on the journey

of what created this

mysterious scar in the Earth's

crust, and how it continues to

mold the planet, takes us

back to 1872, when a British

research vessel, HMS

Challenger, set out on the

first ever mission to map the

ocean floor.

>> Throughout most of recorded

history, men had just assumed

that, beyond a certain level,

the sea was pretty flat,

pretty dead, pretty lifeless.

They weren't expecting to find

anything very interesting.

>> For four years, the

Challenger crisscrossed the

oceans, covering 70,000 miles,

a third of the distance to the

moon.

The crew plumbed the depths

every 140 miles, using a total

of 249 miles of rope, and

hundreds of pounds of

lead weight. It was tedious,

backbreaking work, but at the

time, it was the only way to

measure the depth of the ocean

floor.

When they got to the western

Pacific, 200 miles off the

island of Guam, the crew

routinely lowered the rope for

a measurement.

But the weight kept on

dropping and dropping.

>> It's a big surprise! Nobody

thought the ocean was this

deep. So all of sudden we've

got scientists saying, "Why

is that?"

>> Eventually, the weight

struck the bottom at 4,475

fathoms, nearly five miles

beneath the ocean's surface.

>> The scientists would be

going, "Wow, we've found

something and what does it

mean? Is it a little hole? Is

it a big hole? What kind of

feature is it down there?"

There--there's a whole lot

of questions you get when you

find this one spectacular

reading.

>> The Challenger expedition

marked the birth of modern

oceanography, and provided the

first crude map of the ocean

floor.

It showed how the ocean floor

gently slopes away from the

land, and then plummets

thousands of feet into vast

flat plains. But the western

Pacific is different. It drops

off again, into the five mile

deep hole, a hole that blew

right out of t water the

long-held belief that the sea

floor was flat and

featureless.

And it spawned a mystery,

because nobody could

understand how this strange

underwater feature came about.

It would be 75 years before

any answers emerged. It took a

revolutionary new technology,

sonar, to push the

investigation forward to the

next crucial stage.

Sonar was first developed in

the early 1900s and then

perfected during the 1940s to

detect submarines lurking in

the deep.

The system works by pumping

sound waves through the water.

The waves bounce off solid

objects and are reflected back

to a detector. By measuring

the time it takes for the

sound waves to bounce

back, scientists realized they

could build a remarkably

accurate picture of the world

beneath the waves.

>> The world's major navies

spend a lot of time and effort

developing submarine hunting

technology, then the

hydrographers discover that

you can use this to chart the

bottom of the sea and it's an

awful lot cheaper and easier

than using large numbers of

sailors pulling on ropes.

>> In 1951, a British Navy

research ship returned to the

deep hole found by the

Challenger expedition.

But, this time, they were

armed with sophisticated new

sonar equipment.

And the results were amazing.

Detailed sonar maps revealed

that the deep hole in the

Pacific Ocean floor isn't a

hole at all, but part of a

massive trench, 30 times

deeper than the Empire State

Building is high.

It runs twice the length of

California, 1,500 miles from

the southeast of Guam to the

northwest of the Mariana

Islands.

>> People were probably

astounded by what they were

seeing, because, clearly, the

ocean floor had enormous

changes in relief. It was very

mountainous in some places,

had great deeps in other

places. To a geologist, this

would be extremely exciting.

Even within the trench itself,

there are remarkable

variations. At its southern

end lies the greatest surprise

of all.

The sea floor drops down

another two miles to its

lowest point, a staggering

seven miles beneath the waves.

Scientists had discovered the

deepest part of the oceans.

Even today, it is the lowest

known point on the planet.

They named this part of the

trench the Challenger Deep, in

honor of the ship that

discovered it.

>> To get a sense of just how

deep trenches are, if we take

the heig

we would still have about a

mile of water above us before

we get to the ocean surface.

>> But how the Marianas Trench

was formed remained a mystery.

Investigators decided the best

way to find the answer was to

dive to the bottom of the

trench, to see for themselves

the lowest point on the

planet, the Challenger Deep.

But they faced a major

problem. At the bottom of the

trench, they would have to

contend with pressure a

thousand times stronger than

at the surface, that's the

equivalent of being squeezed

on all sides by the weight of

50 jumbo jets.

To demonstrate the effects of

such pressure, scientists

use a dummy head.

>> Today, what we are going

to do is actually put one of

these Styrofoam wig heads in

the, uh, pressure chamber and

expose it to the, uh, pressure

we would see in the Marianas

Trench. That's about 16,000

psi.

>> A human skull would be

crushed to a pulp, but the

rubbery head will only have

all the air squeezed out.

>> Wow, the head's smaller.

Here's what the original size

was, just for comparison.

[LAUGHS]

Quite dramatic! Pretty stark

difference between, uh,

something that hasn't been

seven miles deep in the ocean

and something that has.

Glad I'm not going there.

[BOTH LAUGH]

At the Mariana Trench, human

life is impossible, we're not

equipped to resist those kinds

of pressures, and so it's

necessary to protect humans

from that type of an

environment.

>> The challenge to engineers

was how to accomplish this.

In 1953, Swiss scientist

Auguste Piccard designed the

Trieste, a pioneering vehicle

that could withstand the

crushing pressures.

The submersible was dominated

by a 50 foot long hull, filled

with light aviation gasoline

and lead weights to control

buoyancy. Slung underneath it

was a tiny six foot spherical

cabin with five inch thick

steel walls.

Finally, after seven years of

modifications and manned test

dives no deeper than three and

a half miles, the Trieste was

ready to attempt the seven

miles to the bottom of the

trench. The commander of this

perilous undertaking was US

Navy Lieutenant and deep sea

explorer Don Walsh.

>> I know the astronauts that

go through this all the time.

"Why do you have to be there?

Why can't we just put up a

robot to do things?" You've

got to be there because that's

what we do.

>> Only a few officers and

scientists knew about the

risky mission, which was

launched in January 1960 from

the western Pacific island of

Guam.

>> Guam in those days was kind

of a backwater, it was just

right for us because we were

trying to do this project sort

of out of sight, because we

weren't too sure it was gonna

work. The navy just didn't

want to be embarrassed by a

failed science spectacular.

>> Accompanying Walsh was the

son of the Trieste's designer,

engineer and oceanographer

Jacques Piccard. The two men

would spend the next nine

hours squeezed inside the

cramped sphere.

>> And we had, erm, 20 cubic

feet of space inside, that's

about the same as a

household refrigerator, and

the temperature was almost

that cold inside. It was a

drama.

>> The story of how the

Marianas Trench came to be is

beginning to take shape.

In 1874, British surveyors

were the first to discover

a five mile deep hole in the

ocean. 75 years later, sonar

mapping revealed the hole to

be a vast, 1,500-mile long

trench, with the deepest part

seven miles beneath the

surface waves of the Pacific.

To gather further evidence,

two courageous men were about

to undertake the most

dangerous dive in history.

They would venture into the

abyss and go to the bottom of

the Marianas Trench.

The Marianas Trench is one of

the most remote, inhospitable

places on Earth.

In January 1960, two deep sea

explorers, Don Walsh and

Jacques Piccard, plunged into

its depths on board the

submersible, the Trieste.

At a speed of just three miles

per hour, they began their

slow descent into the twilight

zone.

By 3,000 feet, the darkness

was total. The only

illumination was from the

Trieste's powerful lights.

>> At the depths we were

operating at, it was always

black. The only thing that lit

up the abyss was the

bioluminescence from animals

and plankton. Like fireflies,

they carry their own light

sources with them.

>> Encased in their five-inch

thick steel sphere, Walsh and

Piccard quickly passed their

test dive record of 18,000

feet. Everything appeared to

be going to plan. At the rear

of the cabin, the crew were

protected by a double layer of

glass. But, two hours into the

dive, the outer pane cracked.

>> We, um, had a great big

bang. We didn't know what it

was. We were at about 20,000

feet, and we looked around and

checked everything,

>> Every square inch of their

tiny life-supporting capsule

was fighting back eight tons

of pressure. With the outer

pane broken, the only thing

between the men and instant

death was a single pane of

glass.

>> If the inner window had

cracked, erm, we would have

been instantly dead, maybe

even before we knew it.

>> But, incredibly, the inner

pane remained watertight.

Walsh and Piccard decided to

continue the descent.

After a tense, claustrophobic

four hours and 48 minutes,

they approached the bottom of

the trench, only to be

startled by movement on the

sea floor.

>> Just before we landed, we

saw a flatfish about a foot

long, and that's a

bottom-dwelling fish, so if

you see one there are others,

>> Nobody expected to see life

at these crushing depths, but

it meant the explorers had

reached their goal, the very

bottom of the Marianas Trench.

The depth gauge, with a

reading of 35,800 feet, nearly

seven miles below the surface,

confirmed the sonar findings.

Squeezed inside their bubble

of breathable air, the two

explorers were closer to the

Earth's centre than man had

ever been.

We took a self-portrait,

that's the picture that you

see. We said we were going

to do it, and we did it.

>> But there was work to be

done. Walsh and Piccard wanted

to make detailed observations

of the enormous trench.

Unfortunately, the Trieste

stirred up a cloud of fine,

powdery sediment from the sea

floor that obscured their

view.

>> WALSH: It was like being in

a bowl of milk at that point.

So, realizing that we weren't

gonna see anything, we decided

to go on back up to the

surface.

>> ANNOUNCER: Off the island

of Guam, the Trieste surfaces

after a descent into the

Marianas Trench.

>> After nine grueling hours

underwater, Walsh and Piccard

returned to the surface on

January 23rd 1960 and

officially entered the record

books for the deepest dive of

all time. To this day, their

extraordinary feat has never

been repeated.

The mission was a success, but

the mystery remained.

Geologists still didn't

understand what could have

formed the immense trench.

And if they couldn't find the

answer inside the trench,

they would have to look

elsewhere.

Perhaps there was something,

somewhere, on the ocean floor

that might explain the

trench's origins.

Throughout the '50s and '60s,

a team of geologists led by

Princeton's Harry Hess

compiled sonar data from all

of the world's oceans.

It was as though they had

pulled out a giant plug, to

drain away all the water, and

expose the ocean floor.

Their maps revealed that the

Marianas Trench is just a tiny

fraction of a network of

enormous underwater canyons

stretching right around the

planet. But that wasn't all.

Running parallel to the

trench, on the other side of

the Pacific, the maps showed a

giant underwater mountain

range, the East Pacific Ridge.

And this too is part of a

global network, a 40,000 mile

long chain of mountain ranges

that ring the globe like the

seams of a baseball, to make

the largest geological feature

on Earth.

It was a major development in

the investigation, one that

scientists hoped might explain

the trench's formation.

The next step was clear.

Investigators needed to

understand whether there was a

connection between the trench

and the East Pacific Ridge.

The breakthrough came from the

unlikeliest of sources. During

the Cold War, the US built a

vast network of underground

seismometers to pick up atomic

bomb testing around the world.

Inadvertently, the

seismometers also detected

naturally occurring

earthquakes. When geologists

plotted these on a map, a

pattern emerged.

The earthquakes were clustered

along the ocean's ridges and

trenches. It was a discovery

that transformed our

understanding of the Earth.

Geologists realized the

friction that cause

earthquakes comes from

movements that must be

occurring deep beneath the

ridges and trenches.

>> With this great investment

in seismology, it became

possible to locate very

precisely where earthquakes

had occurred. And it was these

things, the precise location,

the depth and the motion that

really gave the outlines of

plate tectonics.

>> It was the birth of an

extraordinary new theory. The

solid layer of rock, the

crust, on which the land and

ocean sits, is broken up into

a series of vast slabs, that

geologists call tectonic

plates. It's these plates that

are moving, grinding past each

other, and triggering

earthquakes.

The underwater ridges and

trenches sit on the boundaries

between tectonic plates.

The East Pacific Ridge and the

Marianas Trench lie on

opposite edges of the Pacific

Plate.

The journey to discover what

formed the Marianas Trench is

accumulating additional

evidence. The Trieste dived to

the bottom of the trench,

and confirmed that it is the

deepest point on the planet.

Sonar maps then revealed the

East Pacific Ocean Ridge,

running parallel to the

trench. To solve the mystery

of the Marianas Trench,

investigators needed to find

out exactly what was happening

at the East Pacific Ridge, and

that meant exploring these

vast mountains, 8,000 feet

underwater.

The pieces of the Marianas

Trench puzzle are falling into

place with the knowledge that

it lies on the western edge of

the Pacific tectonic plate.

On the opposite side of the

plate lies the East Pacific

Ocean Ridge, part of an!

enormous chain of underwater

mountain ranges that ring the

globe to create the largest

geological feature on Earth.

Scientists had a hunch that

this colossal ridge might help

explain how the trench was

formed. And they found a major

clue halfway round the globe,

where the ridge passed beneath

the middle of the Atlantic

Ocean.

During the Cold War, the US

Navy developed a new technique

to spot Soviet submarines.

They scanned the seas with a

tool called MAD, a magnetic

anomaly detector, which could

pinpoint steel hulls lurking

in the deep. But they stumbled

across something else. Running

parallel on either side of the

ridge, they found strange

stripes of magnetic rocks,

alternating positive and

negative away from the ridge's

peak.

Here's the Mid-Atlantic Ridge

coming down through here.

Almost perfectly symmetric on

either side of that are these

white and black stripes, these

have often been called zebra

stripes.

>> Geologists know that the

Earth is like a giant magnet

with a north and a south pole.

But the magnetic poles aren't

fixed. Every 300,000 years or

so, the magnetic field

suddenly flips 180 degrees.

When the field flips, a

compass that was previously

pointing north will swing to

the south.

>> This reversing of the

Earth's magnetic field is a

very interesting and exciting

but very puzzling phenomenon

for a geophysicist to explain.

>> Scientists think this

reversal explains the stripes

either side of the ocean

ridge. In the 1960s,

geologists discovered that

molten volcanic rock, known as

magma, swelled up from deep

underground to create the

ridges in the Atlantic and

Pacific.

As magma wells up between the

tectonic plates, it pushes the

sea floor up, and forms the

colossal mid-ocean ridge,

thousands of feet high.

When the rock is hot and

molten, its magnetic minerals

line up with the north-south

direction of the Earth's

magnetic field. As the magma

cools, the minerals are locked

in position.

These rocks act as a permanent

record of the magnetic poles'

location when the rocks were

formed. As more and more magma

is forced up, the old crust is

pushed away from the ridge and

records the reversals in the

Earth's magnetic polarity.

>> If you have reversals of

magnetic polarity, then the

sea floor acts sort of like a

tape recorder and records

these changes in

magnetization, then the

pattern of magnetic stripes

allows people to calculate the

speed at which the plates are

moving apart.

The zebra stripes are proof

that, over time, the sea floor

in both the Atlantic and the

Pacific, is spreading away

from the ridges at a rate of

more than two inches a year.

But geologists needed proof

that magma created the ridge.

If red-hot molten rock is

forming the enormous mountain

range in the Pacific, the

surrounding water should be

warm.

In 1977, a team of scientists

set out to discover whether

this warm water really

existed.

Dudley Foster was the pilot

for these historic dives.

>> It's been an exciting

occupation because you're on

the cutting edge of science,

uh, new discoveries all the

time. Every cruise, there's a

new group of scientists with

new scientific objectives and

there's the exploration and

the discovery and that's

really what puts the thrill in

the job.

>> For weeks, the crew scanned

the undersea mountains without

success. And then, they hit

the jackpot, a bizarre pillar

of rock, spewing hot toxic

gas.

>> And we saw the water was

sort of shimmering, sort of

like, uh, bubbling in a glass

teapot or something.

We stuck the temperature probe

in there and it measured 38,

39 degrees Fahrenheit, which

was really amazing, 'cause

the--the ocean's a huge heat

sink, and so to see something

warm like that was kind of

startling.

>> In these pillars of rock,

the expedition had found the

heat from the magma surging up

from deep inside the Earth.

It wasn't warming the water

evenly along the ridge, it was

channeled up through strange

hydrothermal vents.

>> FOSTER: When you make these

discoveries, you don't know

how significant they are. The

true significance of 'em maybe

takes several years to

appreciate, and this was one

of those times.

>> For the investigation into

the Marianas Trench, these

vents are a decisive piece of

evidence.

They confirm that magma is

continually creating new crust

at the Pacific Ocean Ridge.

And magnetic zebra stripes

prove that old crust is pushed

away from the ridge towards

the other side of the Pacific

Plate, towards the Marianas

Trench. But this presents

scientists with a puzzle.

If new crust is being created

at the ocean ridge, and the

Earth isn't expanding, then

the old crust must be

disappearing somewhere else.

>> The reason that the Earth's

not getting bigger with sea

floor spreading is because the

same amount of sea floor is

being destroyed in the

Pacific.

>> Something in the Pacific

Ocean is devouring the sea

floor. And all the evidence

points to the Marianas Trench.

In the hunt to discover what

formed the Marianas Trench,

scientists now know crust

created at the ocean ridge is

being devoured somewhere and

by something in the Pacific

Ocean.

They suspect the Marianas

Trench is involved. But the

proof would come, not from the

trench, but from these, the

Mariana Islands, a chain of

volcanoes that break through

the ocean's surface 200 miles

west of the trench. Scientists

noticed the island chain

mirrors the trench's exact

shape. This led them to think

the trench was responsible for

the islands' creation.

>> If, uh, you see pictures of

the Marianas Trench, it's

curved, and the line of

volcanoes that it generates is

curved exactly parallel to it,

>> Geologists believe that the

trench formed the volcanoes

via a process called

subduction. Subduction occurs

where two tectonic plates

collide. As they grind past

each other, the heavier plate

is pushed beneath the lighter

plate. The descending plate is

forced down into the Earth's

intensely hot interior, called

the mantle. It takes with it

water and sediment built up

over millions of years.

>> Volcanoes form above

subduction zones not because

the Earth is hotter there but

because this is where we're

taking the water that once was

in the ocean. It gets taken

into the mantle and gets

sweated out, causes the mantle

to melt and this magma is what

then rises and erupts

explosively out these

volcanoes.

>> The water in the sediment

forces magma to swirl up and

push through the plate above.

And when it breaks the

surface, it creates volcanoes,

like the volcanoes that form

the Mariana Islands.

It was subduction that formed

the islands west of the trench

and gave investigators the

breakthrough they'd been

looking for. Because here, at

last, was a process powerful

enough to create the Marianas

Trench. As the descending

plate dives down, it digs into

the mantle. Here, the

colliding plates form a

trench, a giant crease in

the ocean floor.

It seemed that scientists had

finally explained how the

trench was formed.

There was just one problem. A

very large problem. Around the

world, subduction zones cause

violent earthquakes and

catastrophic tsunamis.

>> We know subduction is

happening because of the

active earthquakes and these

are the most devastating

earthquakes. This is the

earthquake that generated e

tsunami in Sumatra, also the

other very large earthquakes

in Alaska and Chile.

>> But the Marianas Trench,

the deepest subduction zone in

the world, hasn't caused a

devastating earthquake since

records began in the 17th

century. Investigators needed

to know why.

>> Ah, that's--that's, uh, the

$60,000 question.

>> They hoped the trench's

shallower western edge might

provide the answer.

Here, they found an intriguing

chain of underwater hills two

miles below the surface of the

sea.

Engineers drilled down into

the hills and collected core

samples.

An$ when the scientists

analyzed the samples, they

discovered the hills were

actually volcanoes, and they

spewed out not lava, but mud.

The fine, powdery mud is made

up of a soft type of rock that

has been ground up in the

subduction zone. It seemed

this soft rock might explain

why there have been no

major earthquakes at the

Marianas Trench.

>> Everybody has a sense of

what a volcano is but not all

volcanoes erupt igneous rocks,

there's some volcanoes that

erupt mud. And a certain kind

of unusual kind of mud in the

Marianas is made out of

serpentine, and serpentine is

a very weak rock and it can be

scratched with a knife or

something like that.

>> Investigators realized the

grinding plates crush the soft

rock to form a lubricating mud

that prevents large

earthquakes. Then the mud

bubbles up to the ocean floor,

where it forms the strange mud

volcanoes found along the

trench's western edge.

>> Other parts of the world,

like the Andes or maybe

Indonesia, you've got two

plates that are grinding

together and the--one

of the plates is quite strong,

and it takes a big earthquake

to rupture that plate

interface. But if these rocks

are weak like they are in the

Marianas, where you've got

these serpentinites, those

are very weak and it doesn't

take much energy at all to get

the two plates to glide one

past the other.

>> At last, geologists had

discovered what created the

Marianas Trench.

50 million years ago, the

Pacific Plate slipped under

the edge of the Philippine

Plate. As it bent and dived

into the Earth's mantle, it

formed the colossal Marianas

Trench. And the plate is still

moving. Like a giant conveyor

belt, the Earth's crust

travels slowly across the

Pacific Plate, from its

birthplace in the East

Pacific Ridge to its

graveyard, 10,000 miles away

in the Marianas Trench.

Today, the Pacific Plate's

movement can be tracked in

real time.

>> Confirmation has come from

GPS technology, where we can

actually put a transmitter on

an island and come back year

after year and actually follow

it moving a few centimeters a

year towards the trench.

>> It's devouring the crust at

a rate of three inches a year,

about as fast as a human

fingernail grows.

Every four million years, it

swallows an area the size of

the United States. By

consuming the crust created at

the Pacific Ocean Ridge,

the ravenous Marianas Trench

is the world's largest

recycling plant.

But there was one remaining

and major piece of the puzzle

to find. Scientists still

didn't know why it is the

deepest trench on Earth.

They suspected the age of the

sea floor at the bottom of the

trench may provide the answer.

>> It turns out there's a

really strong relationship

between the age of the sea

floor and its depth in the

water.

>> In 1999, a team of deep sea

drillers returned to the

trench to collect core

samples.

>> PLANK: One great thing

about drilling this ocean

crust is we actually got

pieces of it. So, we're

holding in our hands here the

material that's actually getting

subducted at the Marianas

Trench, and it turned out to

be 170 million years old. So

we can say with confidence

that's the oldest ocean floor

before it's getting swallowed

up in the mantle at the

trench.

>> But why is this piece of

rock the oldest on the ocean

floor?

>> PLANK: The sea floor at the

Marianas Trench is so old

because it's been so long

since it was born, so it was

born in the equivalent of the

eastern Pacific today and it's

just been going on longer

than--than any other place in

the oceans before it's been

subducted.

>> The Pacific Plate is the

planet's largest tectonic

plate, covering an area 11

times the size of the United

States.

When crust bubbled up at the

ridge 170 million years ago,

it was light and buoyant.

But as it traveled 10,000

miles across the plate, it

cooled and became compact and

dense. Over millions of years,

the dense crust got heavier

and began to sink into the

mantle below.

Scientists realized that,

because the crust at the

Marianas Trench is the oldest

ocean crust, it's also the

heaviest and so has sunk

deeper into the mantle than

any other area of ocean crust.

Here, at last, was the

explanation for the trench's

extraordinary depth. The

picture of the Marianas Trench

is almost complete.

Volcanic islands mirroring the

trench's exact shape lead

scientists to believe it runs

along a subduction zone.

And slippery mud volcanoes

explain why it doesn't create

large earthquakes. But one

question remains unanswered.

Towards the trench's southern

end, the vast chasm drops a

further two miles to its

lowest point, the Challenger

Deep, seven miles beneath the

waves. The question is, what

makes it plunge so deep?

The investigation into the

Marianas Trench has one final

puzzle to solve.

At the trench's southern end,

the sea floor plummets a

further 10,000 feet into a

seven mile deep chasm called

the Challenger Deep. It's the

lowest point on the planet,

but so far, scientists have

been unable to explain why

this one section of the trench

is so deep. Now, they believe

the shape of the descending

tectonic plate may hold the

answer.

>> The Challenger Deep, in

addition, is a little bit

deeper, because of some

peculiarities relating to how

the slab that's going down is

behaving.

>> A narrow slab of crust has

torn away from the Pacific

Plate's descending edge.

>> STERN: Well, it's basically

got to do with how the slab

pushes the mantle out of the

way. Where you have a narrow

slab, like you have at the

Challenger Deep, it can sink

almost vertically, because the

mantle that it's trying to

displace can move around out

of the way.

Investigators have finally

unraveled the mysteries of the

Marianas Trench.

And in the process, they've

made a discovery with

implications that stretch far

beyond the trench itself.

Studying the ocean ridges led

geologists to believe that

magma, welling up at the

ridges, was pushing the plates

apart.

>> How much weight is that--

>> But the exploration of the

Marianas Trench has changed

this idea forever.

>> People used to think that

maybe the magma would kind of

push the plates apart, and

that idea is largely

discounted now.

>> As the ocean crust travels

from the Pacific Ocean Ridge

to the trench, it changes from

a buoyant, red-hot magma into

a colder, denser and heavier

crust. The plate's leading

edge becomes so heavy that it

drags the rest of the plate

along behind it.

>> The heavy cold plates at

the trenches are sinking down

into the mantle and pulling

the plates apart, uh, at the

ridges, and the magma just

passively, uh, fills in the

gaps.

>> The investigation into the

Marianas Trench has

revolutionized our

understanding of how the

Earth's plates move.

We now know a worldwide

network of subduction zones

drag tectonic plates around

the globe, powering the

movement of continents over

millions of years and moving

the very Earth we stand on.

>> The plates that are moving

fastest on the Earth are

the ones that have all the

trenches.

>> The Pacific Plate is the

fastest moving of the nine

major plates on the planet,

because it is surrounded by

dozens of destructive trenches

like the Marianas. They are

consuming the ocean crust

faster than the Ocean Ridge

can produce it. Over millions

of years, the Pacific Plate

will shrink until, some time

in the distant future, the

largest ocean on Earth will

disappear. Australia will

crash into the United States,

reshaping our planet.

Perhaps one day, downtown

Seattle will compete for real

estate with a suburb of

Sydney, Australia. And

all because of subduction

zones like the Marianas

Trench.

But for all its significance,

man has only ever dived to the

bottom of the trench once, and

there are no immediate plans

to return.

>> Imagine asking someone,

"What is the flora and fauna

of California?" and saying

that someone's spent ten

minutes there, picked up two

ants, come back and said

they've sampled California.

That's probably how well we

know the Marianas Trench.

>> To date, less than five per

cent of the world's oceans

have been explored. But only

by returning to the oceans'

very deepest reaches will we

fully comprehend the

incredible forces that recycle

and rebuild our world.

>> The way I like to think of

it is that ocean exploration

leads to new research

questions. And if we don't

have exploration, we don't

even know the right questions

to ask.

It is now known what a

geological wonder the Marianas

Trench is. Since this deep

chasm in the Earth's crust was

first discovered with a length

of rope and a lump of lead

more than a century ago,

evidence has piled up. A

record-breaking dive to the

lowest point on Earth. Giant

undersea mountain ranges with

bizarre magnetic zebra

stripes, proof that the ocean

crust is spreading towards

the hungry Marianas Trench,

lined with slippery mud

volcanoes which prevent

devastating earthquakes.

And the planet's oldest ocean

crust, the reason that the

Marianas Trench is the deepest

point in the oceans.

In the darkest and most remote

place in the world, scientists

have added to their knowledge

about the powerful forces that

contribute to the dynamic

story of our planet.