Original subtitles

Over sixty per cent 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 travelled 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 is 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 a thousand metres down,

where pressure is a hundred times

that on the surface, crushing the whale's lungs

to just one per cent of their volume.

For us to follow the whale,

we need the very latest submersible.

A reinforced acrylic sphere,

with walls 12 centimetres thick,

protects a pilot and our cameraman

from the enormous pressure below

and allows the submarine to dive to

just over nine hundred metres.

With every passing metre, pressure increases and sunlight diminishes

"One thousand feet ...

By three hundred metres it's already very dark

and the temperature of the water is dropping fast.

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 centimetres 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 forty metres long.

Millions of tiny stinging cells, drifting through the sea.

Five hundred metres 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.

Hatchet fish 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.

Hatchet fish, 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 colour 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.

Descend below a thousand metres and

you enter the dark zone.

No sunlight whatsoever penetrates this deep.

The temperature of the water has dropped

below four degrees Centigrade.

The pressure is more than

a hundred times that at the surface.

Life becomes every more sparse.

It's a dark, dangerous world.

Relative to body size, these are the largest teeth in the ocean,

they are so big that their owner can't even close its mouth.

They belong to the Fang Tooth.

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 colour.

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 metre 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 metre 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 midwater, 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 angler fish 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 favourite 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 defence 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 three hundred metres,

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 the 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.

One thousand million tonnes 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 metres

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 metres,

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 a hundred and fifty metres 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.

Travelling 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 midwater above.

At around three hundred metres,

the drop off levels out and

we move out onto the Continental slope.

This stretches for about a hundred and

fifty miles from the coast, sloping in the gentle gradient

down to a maximum depth of four thousand metres.

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 three per cent 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 defence

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 in 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

Iook 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 two thousand metres.

In the cold waters of a Norwegian Fjord

there was a deep sea reef thirty metres high

and two hundred metres 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 three centimetre 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 it's 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 metres 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 metre 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 six gilled shark.

These monsters grow to eight metres long.

Six gills are living fossils.

For a hundred and fifty million years

they have existed unchanged

living in water as deep as two thousand five hundred metres.

Very few people have ever been lucky enough

to glimpse these sharks from submersibles

and we know almost nothing about their behaviour.

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 thirty ton Grey 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 seven metres 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 is 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 is gone.

Eighteen 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 specialised 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 two metre 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 4500 metres, three miles below the surface.

Around 3000 metres 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 seven miles below sea level.

There are just five manned submersibles

world-wide that can reach the abyssal plain

and between them, so far,

they have explored less than one per cent of it.

There are a thousand 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 odour 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 two miles off the sea floor

the ridges extend for over twenty eight thousand miles

the largest mountain chain on earth.

When submersibles finally succeeded in reaching the ridges...

in the 1970's 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 a hundred degrees Centigrade

but down here under the immense pressure of the ocean

it remains liquid at temperatures as hot as four hundred 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 sulphides

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 eighty 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 two metres long and four centimetres 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 specialised bacteria

that are able to derive energy from the sulphides that are pouring from the vents.

The worms' plumes were bright red

with haemoglobin that carries sulphides

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 five hundred 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 ten 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 twenty metres 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 sulphides

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 sulphides.

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 metres.

This new species also uses bacteria

to fix energy from sulphides

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 two hundred 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 ten years

two completely new ecosystems, both totally independent of the sun's energy

has been quite extraordinary.

So far, we have explored just one per cent of the deep ocean floor.

Who knows what is still out there to be discovered?