All language subtitles for 02 The Deepest Place on Earth.Engilsk

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 revolutionise our understanding
of the powerful forces

that shape not just the trench,

but the Earth itself.

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 mould 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, fairly 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'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 the 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 realised 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 honour of the ship that discovered it.

To get a sense of just
how deep trenches are,

if we take the height of Mount Everest,

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,
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, realising 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.

NARRATOR: After nine
gruelling 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 realised the friction
that causes 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 magnetisation,

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 channelled 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
the 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.

And when the scientists
analysed 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 realised 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 centimetres 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 travelled 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 realised 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 unravelled
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 revolutionised 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 5% 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.