The Blue Planet

Dwarfed by the vast expanse of the open ocean,

the biggest animal that has ever lived on our planet.

A blue whale, 30 meters long and weighing over 200 tons.

It's far bigger than even the biggest dinosaur.

Its tongue weighs as much as an elephant.

Its heart is the size of a car,

and some of its blood vessels are so wide

that you could swim down them.

Its tail alone

is the width of a small aircraft's wings.

Its streamlining, close to perfection,

enables it to cruise at 20 knots.

It's one of the fastest animals in the sea.

The ocean's largest inhabitant

feeds almost exclusively on one of the smallest -

krill, a crustacean just a few centimeters long.

Gathered in a shoal,

krill stain the sea red,

and a single blue whale in a day can consume 40 million of them.

Despite the enormous size of blue whales,

we know very little about them.

Their migration routes are still a mystery,

and we have absolutely no idea where they go to breed.

They are a dramatic reminder

of how much we still have to learn

about the ocean and the creatures that live there.

Our planet is a blue planet.

Over 70 percent of it is covered by the sea.

The Pacific Ocean alone covers half the globe.

You can fly across it non-stop for 12 hours

and still see nothing more than a speck of land.

This series will reveal

the complete natural history of our ocean planet

from its familiar shores to the mysteries of its deepest seas.

By volume, the ocean makes up

97 percent of the earth's inhabitable space,

and the sheer quantity of its marine life

far exceeds that which inhabits the land.

But life in the ocean is not evenly spread. It's regulated

by the path of currents carrying nutrients,

and the varying power of the sun.

In this first program, we will see how these two forces interact

to control the distribution of life

from the coral seas to the polar wastes.

The sheer physical power of the ocean

dominates our planet.

It profoundly influences the weather of all the world.

Water vapor rising from it forms the clouds and generates the storms

that ultimately will drench the land.

The great waves that roar in towards the shores

are dramatic demonstrations of its power.

Waves originate far out at sea.

There, even gentle breezes can cause ripples,

and ripples grow into swells.

Out in the open ocean, unimpeded by land,

such swells can become gigantic.

It's only when an ocean swell eventually reaches shallow water

that it starts to break.

As it approaches the coast, the water at the bottom of the swell

is slowed by contact with the seabed.

The top of the swell, still traveling fast,

starts to roll over and so the wave breaks.

The ocean never rests. Huge currents, such as the Gulf Stream,

keep its waters constantly on the move all round the globe.

It's these currents more than any other factor

that control the distribution of nutrients and life in the seas.

A tiny island lost in the midst of the Pacific.

It's the tip of a huge mountain

that rises from the sea floor thousands of meters below.

The nearest land is 300 miles away.

Isolated sea mounts like this one

create oases where life can flourish

in the comparatively empty expanses of the open ocean.

But all the creatures that swim beside it would not be here

were it not for one key factor -

the deep ocean currents.

Far below the surface, they collide with the island's flanks

and are deflected upwards,

bringing with them from the depths a rich soup of nutrients.

Such up-wellings attract great concentrations of life.

Most of the fish here are permanent residents

feeding on plankton - tiny floating plants and animals

nourished by the richness brought up from the depths,

and they attract visitors from the open ocean.

Tuna.

The plankton feeders are easy targets.

All this action attracts even larger predators.

Sharks.

Hundreds of sharks.

These silky sharks are normally ocean-going species,

but the sea mounts in the eastern Pacific

like Cocos, Mapelo and the Galapagos,

attract silkies in huge groups up to 500 strong.

Silkies seem to specialize in taking injured fish

and constantly circle sea mounts

on the lookout for the chance to do so.

But silkies are not the only visitors.

Hammerheads gather in some of the largest shark shoals

to be found anywhere in the ocean.

Sometimes, thousands will circle over a single sea mount.

But these sharks are not here for food.

They have come for another reason.

Some of the locals provide a cleaning service.

Following the last El Nirio year,

when a rise in water temperatures gave many sharks fungal infections,

the number of hammerheads visiting the sea mounts

reached record levels.

Nutrients also well up to the surface

along the coasts of the continents.

This is Natal on South Africa's eastern seaboard.

It's June,

and just off-shore, strange black patches have appeared.

They look like immense oil slicks up to a mile long.

But this is a living slick:

millions and millions of sardines on a marine migration

that in terms of sheer biomass,

rivals that of the wildebeest on the grasslands of Africa.

These fish live mostly in the cold waters south of the Cape,

but each year the coastal currents reverse.

The warm Agulhas current that flows down from the north

has been displaced by cold water from the south,

and that has brought up rich nutrients.

They in turn have created a bloom of plankton,

and the sardines are now feasting on it.

As the sardines travel north,

a whole caravan of predators follow them.

Thousands of Cape gannets track the sardines.

They nested off the Cape and timed their breeding.

So that their newly-fledged chicks

can join them in pursuing the shoals.

Below water, hundreds of sharks have also joined the caravan.

These are bronze whaler sharks,

a cold water species that normally lives much further south.

These three-meter sharks

cut such great swathes through the sardine shoals

that their tracks are visible from the air.

Harried by packs of predators

and swept in by the action of the waves,

the sardine shoals are penned close to the shore.

Common dolphin are coming in from the open ocean to join the feast.

There are over a thousand of them

in this one school.

When they catch up with the sardines, the action really begins.

Working together, they drive the shoal towards the surface.

It's easier for the dolphins to snatch fish up here.

Now the sardines have no escape.

Thanks to the dolphins,

the sardines have come within the diving range of the gannets.

Hundreds of white arrows shoot into the sea,

leaving long trails of bubbles behind each dive.

Next to join the frenzy are the sharks.

Sharks get very excited when dolphins are around.

They can feed particularly well

once the dolphins have driven the sardines

into more compact groups near the surface.

As the frenzy continues, walls of bubbles drift upwards.

They are being released by the dolphins

working together in teams.

They use the bubbles

to corral the sardines into ever tighter groups.

The sardines seldom cross the wall of bubbles

and crowd closer together.

Bubble netting in this way,

enables the dolphins to grab every last trapped sardine.

Just when the feasting seems to be almost over,

a Bryde's whale.

The survivors head on northwards,

and the caravan of predators follows them.

Nutrients can also be brought up, though less predictably,

by rough weather.

Particularly near the poles, huge storms stir the depths

and enrich the surface waters,

and here, in the South Atlantic,

the seas are the roughest on the planet.

And very rich seas they are, too,

for here, the cold Falklands current from the south

meets the warm Brazil current from the north,

and at their junction is food in abundance.

These black-browed albatross are duck-diving for krill

that has been driven up to the surface.

Like all albatross,

black-brows are wanderers across the face of the open ocean.

A feeding assembly on this scale is a rare sight.

Most of the time, the birds of the open sea are widely dispersed,

but these feeding grounds are close to an albatross breeding colony,

and a very special one.

This is Steeple Jason,

a remote island in the far west of the Falklands.

It has the largest albatross colony in the world.

There are almost half a million albatross here,

an astonishing demonstration of how fertile the ocean can be

and how much food it can give

even to creatures that do not actually live in it.

Nutrients by themselves

are not enough to generate these vast assemblies.

The heat and light from the sun is also essential

for the growth of the microscopic floating plants -

the phytoplankton.

And it's the phytoplankton that is the basis of all life in the ocean.

Every evening, the disappearance of the sun below the horizon