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.

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.

Now we can dive these uncharted depths,

to discover what secrets lie beneath.

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 "cock-eyed 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 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 favourite 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 of the sun.

A giant black void

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

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 seabed 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 mudplains

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.

Thirty tonnes of it.

Finally it settles on the ocean floor

and its presence is soon detected.

Sixgills 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

Twenty-four 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 three and a half 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're 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 over-fishing 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 sea water,

it accumulates in great pools on the sea floor.

It's difficult to make sense of the sight.

A lake of concentrated salt water,

fifteen 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 or 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.

It' 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 miniscule 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

than 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 mouth parts.

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.

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.