Original subtitles

Over 60% of our planet

is covered by ocean more than a mile deep.

That, the deep sea, is by far

the largest habitat on Earth,

and it's largely unknown.

Join us on a journey to the very bottom of the deep sea,

to an alien world never revealed before.

It's home to some of the strangest animals on Earth.

Fish flash in the darkness.

New species are discovered on almost every dive.

More people have traveled into space

than have ventured this deep.

Come on a journey into the abyss.

A sperm whale takes a breath, its last for over an hour.

It's about to leave the warm,

well-lit surface waters,

and dive far down into the cold,

dark depths of the deep ocean.

At the surface,

it took in air at the same pressure as we breath it.

But it's going to look for food

at more than 1,000 meters down,

where pressure is 100 times that on the surface,

crushing the whale's lungs to just 1% of their volume.

For us to follow the whale,

we need the very latest submersible.

A reinforced acrylic sphere with walls 12 centimeters thick

protects a pilot and our cameraman

from the enormous pressure below,

and allows the submarine to dive to just over 900 meters.

With every passing meter,

pressure increases, and sunlight diminishes.

1,000 feet.

By 300 meters, it's already very dark,

and the temperature of the water is dropping fast.

My depth is 1,755. Temperature is...

We are entering the twilight zone,

a weird world of gloom, where many animals

have become completely transparent.

In this twilight, an animal needs to see,

and yet, as far as possible,

must avoid being seen.

A giant amphipod, 12 centimeters long,

and almost perfectly transparent.

Its head is completely filled by two huge eyes,

with which it strains to detect its prey.

Another twilight monster, Phronima,

the inspiration for the Alien movies.

She and her developing pink offspring

live like parasites in the stolen body of a jelly.

This impressive cutlery set

and its huge eyes make Phronima

a powerful predator.

Even really complex animals have become transparent

in the twilight zone.

Squids are among the most advanced of invertebrates,

but this one never meets a hard surface in its entire life,

so its body need not be as robust

as that of its shallow-water cousins.

There's a rich variety of jellies

that live nowhere else but in the deep sea.

Thousands of tiny cilia propel them

through a world without walls.

Invisible in the gloom,

they grope blindly for their prey.

Comb jellies let out long, sticky nets

to catch passing copepods.

But the most extensive death trap

is set by siphonophores.

This pulsating bell is the head of a colonial jelly

that can be 40 meters long.

Millions of tiny stinging cells drifting through the sea.

500 meters down,

and in even the clearest tropical waters,

only the faintest vestige of the sunlight remains,

so little that our eyes can't detect it.

But others can.

Survival in the twilight zone

is all about seeing yet not being seen.

Hatchetfish are masters of the game of hide and seek.

They have the large, sensitive eyes

needed for seeking prey,

but their bodies are flat.

And their sides are highly silvered.

Head-on, they are just visible, thin though they are.

But as soon as they turn,

their mirrored sides reflect the remnants

of blue light from the surface,

and they disappear into the gloom.

Viewed from the side,

whole shoals can hide in this way.

But what about from below?

The tubular eyes of many of the predators,

even in this gloom,

are able to distinguish their prey silhouetted against

the scarcely detectable glimmer of light from above.

Hatchetfish, however, have a way of confusing any eyes

that might be searching for them from below.

Their bellies carry rows

of light-producing cells called "photophores."

They can use these to exactly match

the changing color of light from the surface far above.

This counter-shading breaks up their silhouette,

making them almost invisible from below--

almost.

But these are no ordinary eyes.

The enormous yellow lenses enable their owner

to distinguish between light

produced by photophores and sunlight.

So, one device for escape is countered by another

equally subtle one for attack in an evolutionary arms race

that has been waged for millions of years.

Let's begin.

My dog is one of us.

You and me, girl.

Home protection? Hunting.

Out here, most folks let things go.

I guess I'm not most folks.

I want to live.

Are you prepared to kill?

Yes.

One of us is not leaving this island.

You ready?

Descend below 1,000 meters, and you enter the dark zone.

No sunlight whatsoever penetrates this deep.

The temperature of the water

has dropped below 4 degrees centigrade.

The pressure is more than 100 times that at the surface.

Life becomes ever more sparse.

It's a dark, dangerous world.

Relative to body size,

these are the largest teeth in the ocean.

They're so big that their owner can't even close its mouth.

They belong to the fangtooth.

Unlike most deep-sea fish, this has powerful muscles,

and is an aggressive hunter.

With food in such short supply at this depth,

dark-zone predators have to be able to deal with

a meal of almost any size.

Many animals here are dark red, like this deep-sea jelly.

Caught in the lights of the submersible,

it's a spectacular firework display of color.

Normally, no red light penetrates as deep as this,

so animals with red pigment

appear completely black down here,

perfectly concealed.

Predators here, however, don't just rely on vision.

Many have tiny eyes.

Instead, their thin rod-like bodies are lined with organs

sensitive to tiny movements in the water.

This monster, half a meter across,

is a hairy angler.

This is the first time it's been seen.

It's covered with hundreds of sensitive antennae,

each capable of detecting the movements

of any prey careless enough to stray

too close to this motionless predator.

But this surely must be the strangest of all

the deep-sea fish yet discovered.

A highly sensitive meter-long tail

hangs down from the head

that makes up a quarter of its body.

Its eyes are tiny, but its mouth is truly enormous.

It's called the "gulper eel" because it can engulf a meal

of almost any size.

Hanging motionless in mid-water,

its enormous gape enables it to deal with passing prey,

whether it's small or large.

Gulper eels can swallow prey as big as themselves,

which is very useful in a world where you never know

when the next meal is coming along.

Even in the dark zone, there is some light.

Turn off the submersible headlights,

and you see a pyrotechnic display outside.

These lights are created by animals.

This is bioluminescence.

A deep-sea anglerfish flashes in the darkness.

The light is generated by bacteria

that live permanently inside the lure,

which attracts prey to these murderous teeth.

There are all sorts of lures out in the darkness.

"Come into my mouth, little fish."

And what is the purpose of this lure,

suspended on a long rod, way below

its owner's terrifying set of teeth?

It's difficult to be sure, but then, this monster does have

another giant flashing lure much closer to its mouth.

These fish are called "anglers" because they use their lures

in much the same way as fly fishermen

use their imitation flies.

For a hunting squid with huge eyes,

this glimmer is intriguing.

It might just be food.

A satisfying meal for a fish

with a highly extendible stomach.

Attracting a mate in this endless darkness

can be even harder than finding food.

Flashing lures may be helpful in doing this.

Certainly only female anglers have them.

The tiny males are just a tenth the size of the females.

Their only purpose is somehow

to find a mate in the darkness.

She releases chemicals into the water,

which the males scent with a special white organ

in front of their eyes.

Having found a partner, the male bites at her belly

with specially designed teeth.

He needs to get permanently attached.

Within a matter of weeks,

the male is completely fused to the female,

and there he will stay for the rest of his life.

Her blood circulating in his body

provides him with all the sustenance he needs.

In return, she gets a continuous,

reliable supply of sperm--

a brilliant solution to the problem

of finding a mate in the vast emptiness of the deep sea.

To help in the constant battle between predators and prey,

some fish in the dark zone have developed headlights.

These light-producing photophores beneath their eyes

may be used to search out prey in the darkness.

Most bioluminescence in the deep sea

is blue or greenish-blue,

but a very few predatory fish produce red light.

With this, red prey becomes obvious in the darkness.

Red light is rare down here,

and most animal eyes can't see it.

Only these fish can do so.

This gives them a sniper scope,

a headlight invisible to their targets.

This copepod, un-alarmed, takes no avoiding action.

Bioluminescence is useful in escape as well as attack.

A shrimp senses a threat.

It spins in the water, releasing a bioluminescent glue.

This acts like a burglar alarm,

startling the attacking fish,

and leaving it illuminated in the dark,

and vulnerable to its own predators.

These twinkling lights in the darkness

are produced by copepods.

They probably flash like this

to communicate with one another,

and confuse their predators.

The most sensitive eyes in the ocean

belong to an ostracod called "Gigantocypris."

It's the size of a pea,

but that's enormous for an ostracod.

Copepods are a favorite prey, and it actively searches for

their flashes in the darkness.

But this copepod has a way of confusing

a hunting Gigantocypris.

It discharges a packet of bioluminescent liquid.

The flash is delayed, like a depth charge.

Spinning, confused in the water,

Gigantocypris chases after the flashes.

And the copepod slips away, unseen, into the darkness.

The ultimate bioluminescent defense mechanism

has to be the light show created by

the deep-sea jellyfish periphylla.

That, presumably, is the way it scares away its enemies.

These bright lights are all produced by firefly squid.

Normally, they live way down at around 300 meters,

beyond the reach of these Japanese fishermen's nets.

But for a few months each spring,

they come to the surface every night.

The brightest lights come from the bioluminescent tips

of their two front tentacles.

But it's only in the dark of the deep sea that you can

really appreciate the full complexity

of their displays.

It's not just their tentacles,

but their whole bodies, that are covered in photophores.

The exact function is not clear.

The bright tentacle tips may be for attracting mates

or dazzling predators.

The rest may be camouflage,

providing counter-shading for the squid

as they journey up into the twilight zone.

Every night in the season,

hundreds of thousands of squid

journey up into shallow water to spawn.

Before dawn, they will return to the depths,

leaving their eggs to develop in the shallows.

The daily cycle of the sun has a profound influence

on life in the deep ocean.

As the sun sets, it triggers the largest migration

of living organisms on our planet.

1,000 million tons of animals

travel up from the dark zone into richer,

shallower water every night.

Tiny grazers are first up,

searching for the microscopic plants

that only grow in shallow, sunlit waters.

Predators follow the grazers.

An enormous variety of different animals

join the convoy, or feed off it as it passes.

Many will travel up hundreds of meters

towards the surface, and then, at dawn,

finding themselves at greater risk from predators,

the visitors return to the safer darkness of the depths.

The sun's rays only have a direct effect

in the top hundred meters, or so, of the ocean.

It's only here that photosynthesis can take place,

and coral reefs can flourish.

Leave this thin, rich slice of life,

and travel over the outer face of the reef,

and you quickly enter a far more demanding world.

Below 150 meters,

photosynthesis becomes impossible.

You find no plants, just animals.

Here, the animals are adapted to catch marine snow--

particles of dead animals and plants

that drift down from above.

So, they depend, second-hand,

on the energy captured from the sun

by organisms living in the surface waters.

Traveling close to the sea floor,

we're going to take a journey to the very bottom

of the deep sea,

to a world completely separate from the mid-water above.

At around 300 meters, the drop-off levels out,

and we move out onto the continental slope.

This stretches for about 150 miles from the coast,

sloping in a gentle gradient

down to a maximum depth of 4,000 meters.

Water temperatures down here drop below 4 degrees centigrade,

and the pressure can reach up to 400 times that of the surface.

Without the lights of the submersible,

it would be completely dark.

The water is crystal clear

because there's so little organic matter.

Only 3% of the potential food

in the surface waters reaches the continental slope.

At first sight, it appears a lifeless desert.

But take a closer look,

and you notice a network of tracks and trails.

There is life, even down here.

These animals would die immediately

if brought to the surface in nets,

so you can only see them behaving normally

from submersibles.

Many are new to science.

The deep sea floor is dominated

by echinoderms-- sea cucumbers,

brittle stars, and sea urchins.

There are literally millions of them

marching across the seabed, hoovering up

any edible particles there might be in the sediment.

They come in all sorts of shapes and sizes,

and though they are very thinly spread,

the deep ocean floor is so vast that these are among

the most numerous animals on the planet.

Their spikes are good for locomotion and defense,

but perhaps not quite so good when it comes to mating.

Finding a mate in this largely empty sea floor

could be a problem, so some urchins stay together on herds,

to be sure that they're never too far

from a potential partner.

Rocky outcrops provide good anchorage for animals

that rely on food that might drift past.

These crinoids, or sea lilies,

look like plants, but are, in fact, animals.

Their long stalks ensure that their umbrella,

of feeding tentacles, are positioned

to best effect in the current.

Particles are swept onto the arms,

and carried down to a mouth in the middle of the umbrella.

These sudden movements swat away tiny amphipods

that try to steal the sea lily's captures.

Coral reefs are not supposed to exist

in total darkness,

but recently, a new kind of coral

was found as deep as 2,000 meters.

In the cold waters of a Norwegian fjord,

there was a deep-sea reef 30 meters high

and 200 meters long.

This coral gets no energy from the sun,

so it has to be very efficient in catching food.

Its polyps are far larger

than those of shallow-water corals.

These are, in fact,

the largest coral polyps in the ocean.

They belong to the deep-sea mushroom coral.

Their 3-centimeter-long tentacles can catch

far larger prey than other corals can.

This necessity to capture every particle of food

that comes within reach in this near desert

has radically changed many animals.

Most tunicates are filter feeders,

but this one, uniquely, has become a predator,

and its greatly enlarged siphon

has been converted into a trap.

Most sea cucumbers stay firmly on the bottom,

but not this extraordinary deep-sea species.

Its skirts of skin allow it to swim

hundreds of meters above the sea floor.

Eventually, it will descend, and, with luck,

will land on fresh feeding grounds.

This, though, has to be the most extraordinary

animal design of all.

It's a polychaete worm, and normally, you would expect

the long, pulsating body to be stuck firmly in the sediment.

This worm, alone in its group, swims in the open water.

Propelling itself with its yellow frill,

it moves about, and so finds new sources of food,

or maybe succeeds in escaping from a predator.

This is chimaera, a close relative of the sharks,

less than a meter long.

Sensory pits on its chin help it hunt prey on the bottom,

while its surprisingly large eyes may help it

spot bioluminescence.

Large fish are rare down here.

There's simply not enough live prey to sustain them.

Most have become scavengers.

A dead tuna has attracted a deep-sea conger eel...

and a sixgill shark.

These monsters grow to 8 meters long.

Sixgills are living fossils.

For 150 million years, they have existed unchanged,

living in water as deep as 2,500 meters.

Very few people have ever been lucky enough

to glimpse these sharks from submersibles,

and we know almost nothing about their behavior.

The body of a tuna is a substantial meal,

but just occasionally, a really gigantic corpse

drifts down to the deep sea floor.

This is the freshly dead carcass of a 30-ton gray whale.

It's resting on the sea floor a mile down.

It's only been on the bottom for six weeks,

but already, it has attracted hundreds of hagfish.

These ancient scavengers are nearly always

the first to discover a fallen body,

and are attracted from miles around.

They lack jaws, and rasp at the flesh

with two rows of horny teeth on either side

of their sucker-like mouths.

Next to arrive, a sleeper shark,

a real deep-sea specialist.

They grow to over 7 meters long,

and have never been filmed at such a depth before.

The gaping wounds in the whale's flank are its work.

Unlike the hagfish, it has powerful jaws,

so it's able to rip off huge chunks of meat.

Sharks, hagfish,

and a whole succession of different deep-sea scavengers

will feast on the carcass for years

before all its nutriment has gone.

18 months later, when we returned to this whale,

all that was left was a perfect skeleton, stripped bare.

It was almost as if a museum specimen

had been carefully laid out on the sea floor.

At first, the skeleton seemed totally abandoned.

But even after so long,

there was still some flesh left in the head.

Hagfish have a skeleton of cartilage,

and are so flexible that they can tie themselves into knots,

and so get a better purchase on the flesh they feed on.

But smaller organisms had fed here.

A thick band of white bacteria had formed on the mud

outlining the original shape of the whale.

And on the skeleton itself,

colonies of specialized bacteria were extracting energy

from the bones themselves.

Most remarkably, and in huge abundance,

polychaete worms were collecting the last edible fragments.

These are a new species that, so far,

have only been found on the fallen bodies of whales.

Scientists have discovered 178 different animals

on a single whale vertebra,

most of which have been found nowhere else.

This whale, lying over a mile down,

was not filmed from a submersible

with an acrylic sphere.

Such craft can't go as deep as this.

To withstand the pressure here,

you need a far stronger submersible.

This is Alvin, a 2-meter-wide sphere

with just enough room in it

for a pilot and two observers.

Its walls are made of titanium.

The viewing ports have to be tiny.

Any larger, and the submersible

would implode under the enormous pressure down here.

Alvin can dive to 4,500 meters,

3 miles below the surface.

Around 3,000 meters, the continental slope

finally flattens out, and joins the abyssal plain.

This covers over half the Earth's surface.

Mostly, it's completely flat, but in places,

it's gashed by massive trenches hundreds of miles wide.

The deepest of these is the Mariana Trench,

which drops to over 7 miles below sea level.

There are just five manned submersibles worldwide

that can reach the abyssal plain,

and between them, so far, they have explored

less than 1% of it.

There are 1,000 times fewer large animals down here

than on the continental slope,

but in places, hundreds of brittle stars

march over the sea bed in search of food.

Fish have been found right down to the bottom

of the deepest trenches.

Most come from one family, the aptly named "rattails."

They forage near the sea floor,

and use their battery of sensory pits

to follow odor trails from rotting carcasses.

Rattails can travel long distances

across the abyssal plain in search of food,

but others down here prefer to sit and wait.

This is a tripod fish.

It supports itself on two specially adapted fin rays,

and can sit motionless for hour after hour.

It does have tiny eyes, but it's almost totally blind.

It locates potential prey with a pair of fins behind its head,

which are sensitive to even tiny movements.

We know more about the surface of the moon

than we do about the abyssal plain.

Every dive still produces complete surprises.

This deep-sea octopus is about the size of a beach ball,

and has been nicknamed "Dumbo."

An umbrella of skin between its tentacles

and its extraordinary flapping ears

allow Dumbo to hover effortlessly

over the sea floor as it searches for food.

Right in the middle of the abyssal plain

lie the largest geological structures on our planet...

...the mid-ocean ridges.

Rising almost 2 miles off the sea floor,

the ridges extend for over 28,000 miles,

the largest mountain chain on Earth.

When submersibles finally succeeded

in reaching the ridges in the 1970s,

they found an extraordinary world with mile upon mile

of once-molten rock that had welled up

from the deep in the past, and had now solidified.

They discovered towering chimneys

pouring out water as hot as molten lead.

At the surface, water becomes steam at 100 degrees centigrade,

but down here, under the immense pressure of the ocean,

it remains liquid at temperatures

as hot as 400 degrees centigrade.

The submersible has to move carefully.

Disaster is very close when surrounded by

such enormous temperatures and pressures.

And here, where the very water

is loaded with hydrogen sulfides

poisonous to normal life processes,

they found living creatures.

Some of the chimneys were encrusted with white tubes.

The tubes were inhabited

by a new species of polychaete worm

that was exposed to temperatures

as high as 80 degrees centigrade.

No other animal on Earth was known to tolerate

such high temperatures, so the scientists called

these creatures "Pompeii worms."

But this was just the beginning.

Nearby, there were chimneys completely covered

by whole communities of different organisms.

The bottom of the vent was encrusted with large mussels.

There were swarms of white crabs,

and most spectacular of all, dominating the chimney

were hundreds of bright-red tube worms,

each 2 meters long and 4 centimeters wide.

Until these creatures were discovered,

all life on Earth was thought to be dependent on the sun.

But here, in the complete darkness of the deep,

they had discovered a rich density of life

that clearly derived no energy from the sun.

So, what do they live on?

The answer was found within the tube worms themselves.

They were packed full of specialized bacteria

that are able to derive energy from the sulfides

that are pouring from the vents.

The worms' plumes were bright red

with hemoglobin that carries sulfides and oxygen

down to the bacteria.

These bacterial colonies are the primary source of energy

for all the life that lives here.

The mussels were packed with them.

Just as green plants are the basis of life for animals

living in the sun,

so these bacteria and other microbes are

at the foot of the food chain on which over 500 species depend.

Crabs and shrimps feed off bacteria,

and even try to steal pieces of tube worm plumes.

Since the vents were first visited by biologists in 1979,

a new species has been described every 10 days.

At the top of the food chain,

fish that never stray far from the vents.

But they, or their descendants,

will have to move eventually, for we now know

that individual vents are rarely active

for more than a few decades.

Such a density of life living in such harsh conditions

in the middle of a vast and otherwise barren

abyssal plain astounded the biologists

who first saw it.

It seemed to them that here was evidence of how

life on this planet,

which certainly started in the sea,

might have begun.

Deep-sea submersibles made an even more

extraordinary discovery in 1990.

Over half a mile down,

at the bottom of the Gulf of Mexico,

they came across what appeared to be an underwater lake

over 20 meters long, with its own sandy shore.

Around its edge, there even seemed to be a tide line.

But this couldn't be, of course.

This was underwater.

In fact, the lapping edge was created by a thick soup

of salty brine far heavier than the surrounding seawater.

And the sand was made up of

hundreds of thousands of mussels.

Once again, in the midst of a totally barren seabed,

an extraordinarily rich oasis of life

totally independent of the sun's energy.

The source of energy this time was not sulfides,

but methane bubbling out of the sea bed.

And once again, the mussels carried special bacteria

capable of fixing the methane's energy.

Just like the hot vents, a complete ecosystem

had developed based on the bacteria.

There was an enormous variety of completely new species--

shrimps, weird squat lobsters,

and bright-red polychaete worms.

These oases were called "cold seeps,"

and were surprisingly similar to the hot vents.

The geological processes in the sea floor

that produce methane also tend to result in

the release of hydrogen sulfides.

It was hardly surprising, then, when,

not far from the brine pool,

they found tube worms.

Extensive fields of tube worms

that stretch for hundreds of meters.

This new species also uses bacteria

to fix energy from sulfides,

but it extracts them directly from the ground.

Their beautiful gills are only used

to supply oxygen to the bacteria.

Amazingly, these tube worms are over 200 years old.

While hot-vent tube worms are thought to be

the fastest-growing invertebrates in the sea,

these appear to be far slower.

All the more reason to protect your gills

from biting amphipods.

The energy sources exploited by the hot-vent animals

may suddenly fail, but here,

life can enjoy a more stable geological future.

To discover, within 10 years, two completely new ecosystems,

both totally independent of the sun's energy,

has been quite extraordinary.

So far, we have explored just 1%

of the deep ocean floor.

Who knows what is still out there to be discovered?