Original subtitles

Antarctica.

The coldest, the harshest and the most remote continent on Earth.

No human being has ever descended into the depths that surround it...

..until now.

INDISTINCT RADIO CHATTER

The deep ocean is as challenging to explore as space.

We know more about the surface of Mars than we do about the

deepest parts of our seas.

RADIO CHATTER

Now we can dive these uncharted depths to discover

what secrets lie beneath.

INTENSE CREAKING

Sinking down beside the submerged wall of an iceberg,

we enter an unforgiving world.

These waters are the coldest on Earth.

As we descend into the deep, the pressure increases relentlessly.

And the light from above all but disappears.

Yet, incredibly...

..there is life here.

We might have expected that, deep beneath the surface of

the polar seas, the waters would be truly barren.

But in fact we find life here in unimaginable abundance.

Nor is such great abundance confined to Antarctic waters.

Currents carry this richness into the depths of almost every ocean

around the world.

Astonishingly, in the deep sea,

there is more life than anywhere else on Earth.

The sunlight fades and the seas darken.

Here in the Pacific, 200 metres down,

we enter an alien world.

The Twilight Zone -

a sea of eternal gloom.

There are strange creatures here.

A pyrosome.

A tube of jelly two metres long that dwarfs a visitor from above -

an oceanic whitetip shark.

Only a tiny amount of light filters down this far.

Survival here means making the most of every last glimmer.

A swordfish.

Its eyes are as big as tennis balls,

to help it see in the perpetual dusk.

A squid,

but this is one that lives only here.

Its right eye looks permanently downwards.

But its left eye is much bigger and trained upwards to detect the

silhouettes of prey swimming nearer the surface.

No wonder it's nicknamed "the cockeyed squid".

And even stranger.

This is barreleye...

..a fish with a transparent head

filled with jelly so that it can look up through its skull.

We now know that the Twilight Zone is a refuge for an incredible

90% of all fish in the ocean.

Only at night do vast shoals of lanternfish migrate to the surface

to feed on tiny plankton.

By day, they retreat back down here.

Humboldt squid.

Two metres long and 50 kilos in weight.

Like most squid, they're voracious hunters.

There are hundreds of them.

They've found a shoal of lanternfish,

hiding 800 metres down, off the coast of South America.

Their tentacles are armed with powerful suckers

with which they grab their prey.

And when there are no more lanternfish to be found...

..they turn on each other.

This squid has caught a smaller one in its tentacles.

To hide its capture from the rest,

it releases a smokescreen of black ink.

But then an even bigger one challenges it...

..and steals its catch.

The Twilight Zone is the Humboldt squid's favoured hunting ground.

They seldom go deeper...

..into the world of perpetual blackness below...

..The Midnight Zone.

Two thirds of a mile from the surface, beyond the reach the sun.

A giant black void,

larger than all the rest of the world's habitats combined.

INDISTINCT RADIO CHATTER

There's life here...

..but not as we know it.

Alien-like creatures produce dazzling displays of light.

Nearly all animals need to attract mates and repel predators.

This language of light is so widespread here that these signals

are probably the commonest form of communication

on the entire planet.

And yet we still know little about them.

Hunters illuminate themselves, and by doing so

attract inquisitive prey.

This is fangtooth.

It has the largest teeth for its size of any fish.

There are pressure sensors all over its head and body which can detect

anything moving in the surrounding water.

It's the Midnight Zone's most voracious fish.

But prey use light as a distraction.

A decoy of luminous ink.

Down here, in this blackness...

..creatures live beyond the normal rules of time.

Siphonophores are virtually eternal.

They repeatedly clone themselves...

..some eventually growing longer than a blue whale.

Down here it snows.

Continuous clouds of organic debris

drift slowly down from above.

This is food, and a whole variety

of filter feeders depend on it.

Jellyfish...

..and delicate sea cucumbers.

The 1% of marine snow they miss

eventually settles on the sea floor.

Over millions of years it forms a layer of mud

up to a mile thick.

It's an empty plain that covers

half the surface of our planet.

The deep sea bed may at first appear lifeless...

..but it's home to a unique cast of mud-dwellers.

The sea toad.

It is an ambush predator with an enormous mouth...

..and infinite patience.

This fish has been living for so long here

that its fins have changed

into something more useful.

Feet.

They help it shuffle about on the sea floor.

The flapjack octopus.

It hovers just above the surface of the mud as it delicately sifts

through it, searching for worms.

But it can jet away at the first sign of danger.

A sixgill shark as big as a great white.

It may not have eaten for an entire year.

It patrols the mud plains

using a minimum amount of energy.

High above, the carcass of a huge

sperm whale is slowly decaying.

This will be a bonanza for the creatures of the deep.

Food, 30 tonnes of it.

Finally, it settles on the ocean floor...

..and its presence is soon detected.

Sixgill sharks have an exceptionally acute sense of smell.

Just 25 minutes after the whale's carcass arrives...

..a sixgill finds it.

Each bite releases blood into the current.

The news that food is here spreads quickly.

Two more ravenous sixgills arrive.

Within 12 hours,

there are seven enormous sharks

jostling with one another as they compete to tear off mouthfuls.

No-one is prepared to back off.

24 hours later and a third of the carcass has gone.

The first arrival has gorged until it's completely full.

This single meal may be enough to sustain it for a whole year.

Now the clean-up team arrives.

Spider crabs carrying coral in their hind legs,

presumably as makeshift body armour.

There are rock crabs here, too.

They probably detected the carcass almost as soon as the sharks...

..but they can't move as fast.

A month on, and over 30 species of

scavenger are clearing away the last

edible fragments.

But now the scavengers are attracting their own predators.

Scabbardfish, habitually swimming upright,

are picking them off one by one.

Some of the whale's teeth have been dislodged as the skeleton starts to

fall apart.

Four months later, there is nothing left but a few bones.

But even they are food...

..for something.

Zombie worms.

They tunnel into the bones by injecting acid...

..and so reach the tiny amounts of fat that still remain there.

It may take decades, but eventually

the last of the bones will crumble

and the whole 30-tonne carcass

will have been recycled.

A whale fall is a temporary oasis

in the desert of the sea floor.

But there are permanent oases here, too.

Rocks projecting above the mud

provide anchorage for deep-sea corals.

As far down as 3.5 miles,

there are more species of coral in the deep

than on shallow tropical reefs.

Without sunlight,

they rely solely on food drifting in the current.

And they grow just a hair's breadth a year.

But some of them can live for 4,000 years.

They, like their shallow water relatives,

provide homes for all kinds of other creatures.

Growing among the corals is one of the most beautiful of sponges.

This is Venus' flower basket.

These sponges have lodgers.

Shrimps.

There are plenty of predators on the reef,

so the shrimps are fortunate.

Both this male and female were swept into this sponge

when they were tiny larvae,

along with the minute particles of

food on which the sponge feeds.

They found each other and have been here ever since.

Now they're full-grown

and the female is carrying eggs.

Once hatched, the larvae will swim out through the sponge's walls.

But the shrimps will never leave.

They can't.

They are now far too big to go out the way they came in,

and no doubt they will live longer here than they would

if they were wandering about on the reef unprotected.

But how one of the simplest of all animals, a sponge,

is able to build such a complex structure,

to the great benefit of the shrimps...

..is a mystery...

..and surely a marvel.

But today their timeless world is being reduced to rubble.

As overfishing empties the surface waters of the seas,

trawlers have started to ransack the deep.

Now countless numbers of the reefs

that have flourished here for millennia

lie in ruins.

Over time, organic matter on the sea floor slowly decays...

..producing methane.

In the Gulf of Mexico

these eruptions also release a super-salty liquid.

Brine.

Five times heavier than seawater,

it accumulates in great pools on the sea floor.

It's difficult to make sense of the sight.

A lake of concentrated saltwater, 15 metres deep

at the bottom of the sea.

Around its margin, perhaps even more strangely,

there is a profusion of life.

Giant mussels, that can live and grow for a century or more,

pack tightly together,

dwarfing the shrimps and squat lobsters that feed around them.

Cutthroat eels, scavengers,

come to the shores of the brine lake in search of something edible.

Some even venture into the brine.

Spending too long in it

can send an eel into toxic shock.

Its only hope is to rise above it.

It manages to escape.

Others are not so lucky.

The brine embalms their bodies...

..and the casualties of decades

accumulate around the margins.

But parts of the deep are even more hostile.

In places, gigantic cracks stretch for many miles

across the ocean floor...

..canyons that plunge towards the centre of the Earth.

Scans from survey vessels make it possible to graphically reconstruct

an image of this vast submarine landscape.

The deepest of all, at almost seven miles,

is the Mariana Trench in the Pacific Ocean.

Even Mount Everest could disappear inside it.

Down here, in these deep ravines,

it was once thought that nothing whatever could possibly survive.

But there is life even here...

..a kind of sea slug.

A so-called "sea pig".

They, and other simple creatures, manage to survive on the

minuscule amount of food that drifts down here.

Like this starfish,

they can withstand pressure equivalent of 50 jumbo jets

stacked on top of one another.

A remote camera probe reveals

the most extraordinary discovery of all...

..the ethereal snailfish.

At five miles down,

this is the deepest living fish so far discovered.

No-one imagined that an animal as

complex as a fish could exist

in such extreme pressures.

From the greatest depths to the uppermost limit of

the Twilight Zone, it seems that there is nowhere in the deep sea

where life of some kind can't survive.

And we now think that the deep sea may well be

where life on Earth began.

Here, in a world hidden within

the greatest geological feature on Earth...

..running right down the middle of the world's oceans,

an underwater mountain range, spanning the entire globe.

The Mid-Ocean Ridge.

In the South Pacific,

the ocean floor is being torn apart.

Over three quarters of the planet's volcanic activity

occurs in the deep...

..almost all of it along the Mid-Ocean Ridge.

But from this titanic violence come great riches.

Gases and scalding water gush up through the crevices.

Minerals condensing from these jets build up great chimneys...

hydrothermal vents.

This one, 30 metres tall,

has been named Godzilla.

Astonishingly, we now know

that they hold as much life

as tropical rainforests.

In places, half a million individual animals

are crammed into a single square metre.

They depend entirely for their food on bacteria.

And THEY feed on chemicals dissolved in the searingly hot fluid.

Crabs consume the bacterial mats that coat their shells.

Others maintain bacterial cultures

actually within their bodies.

Shrimps carry such cultures in their mouthparts,

but that is a strategy fraught with danger.

To provide sustenance for these microbes

the shrimps must dash into the hot vents...

..and that risks being boiled alive.

In the last decade

the number of hydrothermal vents discovered has doubled.

Every one has its own unique character and community.

But perhaps the most important one of all is in the Atlantic.

It has been named "The Lost City".

Within its 60-metre towers,

something truly extraordinary is taking place.

Under extremes of pressure and temperature, hydrocarbons -

the molecules that are the basic component of all living things -

are being created spontaneously.

Indeed, many scientists now believe

that life on Earth may have begun

around a vent like this,

four billion years ago.

We now know that there are deep seas

on the moons of Jupiter and Saturn.

If life can exist under such

extreme conditions down here,

then surely it could exist

somewhere out there.

The team spent more than 1,000 hours filming in the deep ocean,

mostly from the research vessel Alucia and her twin submersibles,

Deep Rover and Nadir.

Their most ambitious mission was to Antarctica,

hoping to film life two thirds of a mile down,

something never attempted before.

We honestly do not know what we're going to find down there.

We're going to a place that has never been explored.

There could be nothing, there could be a carpet of life,

there could be anything in between. Who knows?

It's a huge technical challenge.

The water temperature here can reach minus 1.8 Centigrade.

INDISTINCT RADIO CHATTER

No-one knows for sure how the subs will cope

with this extreme environment.

OK. Right, I'm going to soak it up. OK.

Just half an hour into the very first dive,

a puddle is forming on the floor of the sub.

Orla confirms it's seawater.

Yeah, Roger. I'm just try to soak up this puddle of water,

and then see if any more comes.

They must find the leak and repair it fast.

Are you going to knock that over my...? Yeah.

You're at 450 metres in a small bubble and water's coming in.

That's a half an hour straight shot back up to the surface.

You're kind of thinking about,

"Are we going to fill up with water? And if we are, there's no way out!"

Yeah.

The sub pilots are well drilled for emergencies.

INDISTINCT RADIO CHATTER

Just pass them over. Yeah.

Ralph quickly finds the flood and fixes it.

He made it all seem absolutely ordinary and normal and,

"I've got this covered, don't you worry,"

and within 20 minutes, he did.

The waters of the Antarctic Sound are potentially rich

but also treacherous.

The Sound is ominously known as "Iceberg Alley".

We've got to find a place where we can get the submarines down and up

safely without any icebergs coming along and bowling them over.

I've got a feeling it's going to be a bit like a game of Space Invaders.

A metre cube of ice weighs a tonne.

You start sort of grinding that around the sphere, it's delicate.

It's like a big Faberge egg.

One dive brings them right up to the underside face of an iceberg.

There are icebergs up there that are the size of a small car, and

then there are icebergs up there that are the size of Hyde Park.

Enormous.

Conditions here can change in an instant.

The captain radios down to the subs.

Yeah, Roger that.

We've currently got a couple of big icebergs coming down

the channel, and it looks like they're on a collision course.

The impact of two icebergs colliding overhead is clearly heard.

INTENSE RUMBLE

That is ice.

With icebergs colliding above and the weather turning fast,

the subs are quickly recalled.

Once again their efforts are thwarted.

Finally, after two weeks, conditions are just right.

Once again the team attempt their 1,000-metre dive in Iceberg Alley.

An hour after leaving the surface they close in on their goal.

999.

Control, control.

This is Nadir

on bottom, depth - one-zero-zero-zero metres.

Roger. Depth - 1,000 metres.

Control out. New record!

First manned sub dive to 1,000 metres in Antarctica!

At the bottom of the ocean, at the end of the world,

the amount of life they find is astonishing.

But they are equally astonished to find that,

two thirds of a mile from the surface,

icebergs are still a danger.

Rocks can drop from them as they melt,

and one lands right in front of the sub.

I don't think many people who are diving subs

ever consider big lumps of rock landing on them.

It's, it's not your normal risk.

If it had hit the sphere,

there's a good chance it would have put a nice scratch down it.

If something of ten, 15, 20 tonnes had hit the sub,

it would completely destroy it.

But over the following dives

the team learns it's these very drop stones

that enrich the Antarctic deep sea bed,

creating firm anchor points for life to thrive.

Proof that the only way to fully appreciate the complexity and

abundance of life in the deep is to go there ourselves.

Next time, we travel to bustling coral reefs.

Here, animals must go to extraordinary lengths

to get ahead of the competition in these crowded cities.