Frozen Seas The Blue Planet

The frozen seas are worlds unto themselves.

Beneath their ceiling of ice,

they have an eerie stillness, cut off from the storms that rage above.

In the winter, the feeble slanting rays of the sun bring little warmth

and the temperature seldom rises above minus 50 degrees centigrade.

For much of the year, it is dark and cripplingly cold.

Yet there is life here at both ends of the earth -

the Arctic and the Antarctic.

For most animals, whether they live in or out of water,

the winters, when much of the sea is frozen,

bring the greatest challenge.

The northern hemisphere.

It's February, and as the Earth tilts on its axis,

the sun's rays creep slowly northwards

and the Arctic emerges from its harsh winter.

The Arctic is a frozen ocean surrounded by continents

and when the surface of the sea freezes from shore to shore,

land predators walk out onto it to hunt.

It's early March and the sea is still covered with ice.

But there are patches of open water - polynyas - that never freeze over.

Here, where tidal currents are squeezed between islands,

the water movement is so strong that ice cannot form.

Walruses spend the winter in polynyas.

Here, they have permanent access to the air,

but they can also retreat to the sea to shelter, to hunt.

Other sea mammals overwinter in the polynyas as well.

In this one, a young bowhead whale.

Here, the current is really fast and the shifting ice is dangerous.

This whale became trapped when ice encircled it last autumn.

There is no food here, but a whale must breathe

and the only place that it can do so, for miles around,

is in this tiny hole.

It's living entirely on its reserves of fat,

but now they are dangerously low.

It will be some months yet before it can escape.

Elsewhere, other whales have also been trapped.

These are belugas. Their tiny hole in the ice has been kept open,

not by currents, but by the belugas' continuous movements

as they rise to breathe.

Open water is now some 20 miles away.

It will be two months yet before the ice melts.

The belugas are extremely thin and most of them are horribly scarred.

But their wounds were not inflicted by the ice.

A whale would be a huge prize for any meat-eating hunter

and these belugas, trapped by the ice,

are within reach of polar bears.

Aware of the danger, the belugas stay submerged as long as they can.

But they can only hold their breath for about 20 minutes.

Catching a four-metre long whale that weighs one ton is no easy task,

even if that whale is weakened by starvation.

But a beluga is well worth waiting for.

Day by day, as the hole gets bigger,

it becomes increasingly difficult for the bear to land a whale.

Keeping its fur in good condition and free from salt

is important for warmth, and the bear uses snow like blotting paper.

These belugas have been attacked by many bears over the last six months

and some have been caught.

It may have taken a long time and a lot of patience,

but a catch, when it's made, brings abundant rewards

of energy-rich blubber.

Gulls rely on bear-kills at this time of the year

and the colour of blood staining the ice

attracts them from a long way away.

The remaining belugas still have a long wait

before they are released from their prison and the threat of slaughter.

In late March and into April,

female bears emerge from winter dens with their cubs.

The mother has not eaten for at least five months.

She's hungry. Very hungry.

This is a crucial time for the cub.

By watching its mother hunt and copying her actions,

it begins to acquire the rudiments of its own hunting skills.

Play is also important

for developing muscles and co-ordination.

As the days go by,

the sun rises higher and remains above the horizon.

The female bear continues to hunt

until her cub is too tired and can't keep up.

She's smelt something.

The pup escapes through a hole in its lair

that leads to the sea below.

Only one in 20 hunts is successful.

The mother must find a seal pup soon

if her cub is not to starve to death.

As spring turns into summer,

the sun's heat begins to melt the sea ice.

Now the ocean is accessible and the Arctic's summer visitors return.

Migrating birds arrive from the south to nest and feed on sea food

that is now within their reach.

Brunnich's guillemots are the northern equivalent of penguins.

But they have the power of flight to reach cliff ledges

where their nests will be safe from predatory bears and foxes.

Nonetheless, they are as at home in the water as they are in the air.

They dive down to a depth of 50 metres or more to catch fish.

In June, the ice begins to fracture.

Cracks form useful corridors of open water for air-breathing animals.

Belugas migrating to their feeding grounds

can penetrate the ice-covered seas to reach areas

where their preferred food - arctic cod - has spent the winter.

Males regularly dive to about 500 metres to find fish.

The females and young, which have smaller lungs, only go to about 350.

In late June and July, narwhals arrive.

The females, who usually lack tusks, come first with their new calves.

The males follow a little later.

They also move up the leads in search of fresh feeding grounds.

Bowheads. Up to 18 metres long and weighing 100 tonnes.

These are the only large whales that stay in the Arctic all year round.

They're not after fish. They're seeking smaller prey.

Despite having the largest mouths - the size of a small garage -

they eat tiny crustaceans - copepods,

straining them from the water with the four-metre strips of baleen

that hang from their upper jaws.

In the summer, they store enough energy to last them until winter

when food will be less abundant.

As the ice melts away,

the polar bears are forced to head for land.

They're excellent swimmers

and can cover 100 miles of open water if need be.

Off east Greenland, there is little ice left by August,

so walruses haul out to rest on land.

They're moulting,

getting rid of their old, parasite-ridden skin.

WALRUSES GRUNT

A bathe in the cold water brings some relief from the itching.

But even there, the odd scratch is irresistible.

They make daily excursions out to deeper water.

Down at 20 metres, they root around in the sediment

using their bristles to search out soft-shelled clams.

Once they find a clam, they suck its flesh from the shell

with their powerful, muscular mouths.

Walruses can feed for five minutes this deep

before they have to return to the surface to breathe.

Elsewhere in the Arctic,

belugas are gathering in their thousands.

They congregate in just a few large estuaries.

Belugas of all ages and sizes come here.

There are even young calves.

Some are so young - born only a week or so ago -

that they need help.

They swim on their mothers' backs to breathe more easily.

As the tide moves up the estuary,

the belugas follow, swimming into shallow water.

Like walruses, they also need to moult.

A combination of warm, fresh water

and vigorous rubbing against the gravel does the trick.

They remain here for days or even weeks,

so it's likely that socialising

is also important to them.

After moulting, they head back out to sea to feed.

It's now autumn and the sea begins to freeze over once again.

Thin sheets of ice form at the surface

and pile up layer upon layer,

gradually creating an impenetrable barrier.

By late November,

the Arctic ocean is sealed once again by ice.

The lights of the aurora play in the winter sky.

At the other end of the planet in the Antarctic,

there is the southern aurora.

Antarctica is now emerging from winter.

This is the coldest, windiest place in the world.

Temperatures are hovering at a numbing -50.

The returning sun has very little warmth.

Very few animals can survive such extreme conditions.

But emperor penguins can.

Standing on the frozen sea, they endure the full force of the storms.

Only by huddling together can they survive the appalling winter months.

They take it in turns to bear the brunt of the gales.

They can only live here at all

because Antarctica is surrounded by the great southern ocean.

No land predators have reached it

so, unlike Arctic animals, they are not threatened by polar bears.

The sea is still frozen

but one seal, nonetheless, manages to stay here

even throughout the winter.

The Weddell seal.

Underwater, it's protected from the storms above

but it must have access to the air all year in order to breathe.

And they keep their breathing holes open with their teeth.

Only by continually scraping away at the ice

can they maintain access to the air.

That means their teeth get worn down.

Then they can no longer hunt or eat effectively.

Weddell seals die young.

The continent of Antarctica is so isolated and so high -

almost 5,000 metres in places -

that it's considerably colder than the Arctic.

Ice slides slowly down from its centre towards its rim

in immense glaciers.

During winter, the continent effectively doubles in size

as the sea freezes over.

Ice forms around its shores

and extends outwards for hundreds of miles around the entire land mass.

Under the sea ice live small, shrimp-like creatures.

Krill. They have been here all winter.

During these dark months,

they feed by scraping algae from the ice.

Remarkably, they also shrink in size

and revert to their juvenile form to save energy.

As the temperature rises in spring, the ice begins to melt

and little air bubbles are released.

Microscopic algae grow around the bubbles

and the krill graze on them, gathering them up with their legs.

As the sun's rays grow stronger

and penetrate deeper into the water, floating algae begin to flourish.

The krill leave the dwindling ice

and gather in swarms to harvest this new crop.

Far to the north, beyond the blanket of sea ice,

chinstrap penguins have been overwintering in the open ocean.

An iceberg gives them the chance of a rest -

if they can get on it.

THEY SQUAWK

But at this time of year,

where they really want to be is on land.

It's getting there that's tricky.

It's spring, and the penguins are returning to breed.

Their need to get ashore is now urgent and imperative.

Doing so is a matter of timing -

and picking the right wave.

But their journey has only just begun.

Most of them will have to walk many miles in order to find a nest site.

PENGUINS CRY

This is Zavodovski Island,

which has the largest penguin colony in the world.

About two million chinstraps breed here and they come to this island

for a good reason.

It's an active volcano. The heat from the crater and the fumaroles

keeps the slopes free from the ice and snow,

allowing these chinstraps to breed earlier than those further south.

But then again, living on an active volcano is not without its risks.

Unlike the emperors, these penguins can lay eggs on the bare ground.

Little wonder so many of them brave the mountainous waves to get here.

Further south, near the continent,

the blanket of sea ice is beginning to break up.

Icebergs are gigantic fragments of ice

that have broken off the front of glaciers.

Over winter, they were frozen into the sea ice,

but now they are adrift once more.

As the bergs break up, they form brash ice.

It litters the backwaters of the Antarctic Peninsula.

WHOOSHING

Minke whales make their way into these placid waters in summer.

This is the most abundant whale in the Southern Ocean.

Minkes are one of the smallest of all the baleen whales.

Like all others, they come here to feed.

The majestic humpback whales are also summer visitors.

They have come thousands of miles from their tropical breeding grounds

to gather the food that is available here in summer.

In four months,

they accumulate enough fat

to provide energy for the rest of the year.

All these animals have come here in search of one thing -

the krill.

Krill is the mainstay of the Antarctic food web.

It occurs in phenomenal quantity -

billions of individuals in one swarm,

and swarms can stretch for miles.

Fur seals also collect this rich, superabundant food.

Krill swarms are very patchy,

but once found, feeding is easy.

Humpbacks engulf hundreds of thousands of them

in a single gargantuan mouthful.

When the going is good, the whales feed continuously,

each eating up to two tonnes of krill in 24 hours.

Further south, near the continent, the sea ice is still sound.

The ice remains for most of the summer.

Emperor penguins make their home here.

These have been feeding out at sea

and are now ready to return to the colony to feed their chicks.

Instead of going straight for the ice edge,

the penguins hesitate some distance away.

They are nervous.

They dive down and investigate the ice edge.

And for good reason.

Leopard seals patrol this border.

Leopard seals are the Antarctic's equivalent of polar bears.

They are the top predators, but they hunt most successfully in the water,

so by and large, the animals they prey on are safer out on the ice.

RATTLING WHINE

They have a lazy grace that belies their ferocious nature.

Confident that the coast is clear,

the emperor penguins head for the ice.

But they don't linger.

Now they have a long walk back to the colony.

Emperor colonies are set back from the ice edge.

In winter, they may be 100 miles from it,

but as summer progresses and the ice melts,

the edge comes ever closer to the colony.

When the chicks are ready for their first swim, the water is close by.

This colony is in the lee of a headland

and that prevents the ice from being broken up by ocean currents.

The returning adults are so full of food, they can barely walk.

But no predator threatens them now.

They can take their time.

HIGH-PITCHED CRIES MINGLE WITH SQUAWKS

Somehow, in this melee of 60,000 or so penguins,

a parent has to find its chick.

It returns to the place where it last left its chick

in the hope that it might still be close by.

But chicks tend to wander,

so the adult has to call to it.

The chick responds

and they home in on one another.

The chick's cries stimulate the adult

to regurgitate some fish.

With the return of one parent, the other is free to feed for itself.

Aware of the leopard seal's presence,

the penguins press together at the ice edge,

unwilling to be the first to dive in.

Occasionally, the seal comes onto the ice and attempts to grab one.

But its most successful strategy by far is to lie in wait.

It hides behind a corner of ice.

The penguins decide to make a dash for it.

The first wave of penguins escape.

Once in open water, they will be safe.

But the seal is alerted by the noise,

and makes its attack.

Almost invariably, it makes a kill.

Encouraged by the absence of the seal,

the remaining penguins make a break for the open sea.

In time, their chicks will fledge.

And when the Antarctic autumn is near its end,

these adults will walk across the newly formed ice,

to endure yet another winter on the frozen sea.

Of all the ocean habitats,

the Poles have to be one of the most demanding places in which to film.

Not only do you have the problems of keeping filming equipment working

in temperatures well below zero,

but you also have your own survival to consider.

Ice may be beautiful, but it's treacherous.

Pieces can break off, leaving you drifting in the freezing seas.

Diving under the ice requires special skills.

Cameraman Doug Allan has those skills in abundance,

having spent much of his life filming around the Poles.

On the Arctic Islands of Svalbard,

Doug Allan and polar expert Jason Roberts

are in search of polar bears.

They are carrying everything they need to live up here for four weeks.

But almost straight away, they run into a problem.

We've got "white-out" - we've got no contrast.

The next route lies out over the sea ice and we need good conditions.

So we've decided to stop here at this snow-covered cabin

in the hope that the weather gets better.

And the snow covering the cabin wasn't just on the outside.

Cup of tea, stage one!

Cabins like this one are set up by hunters and fishermen,

who use them in season.

They provided a vital base camp for our film makers.

Near the shore line of Svalbard, the ice is moving.

Polar bears come here to try to catch seals.

To find a white bear in this white wilderness requires persistence.

Brrr. 11 hours.

I feel as though these things have been glued on my eyeballs.

One bear - but just much too wary of us to let us get anywhere close.

A long day for not very much.

Several months earlier, filming in the Antarctic,

Doug had had better luck finding the animals.

Emperor penguins are regular commuters.

Every day, they return from feeding out at sea

using exactly the same exit point along the ice edge.

Keep rolling!

Still coming!

A remote camera under the ice

helped us predict exactly when the penguins would emerge.

Penguins may be more predictable than polar bears,

but survival logistics in Antarctica are very demanding.

With the nearest civilisation a thousand miles away,

everything you need has to be brought in.

An Italian Antarctic scientific research team

kindly flew in a special camp which was set up near the ice edge.

Back in the Arctic, the weather is still holding things up.

This is definitely not a day to be looking for bears.

It's about minus 15. Wind chill is something ferocious.

I'm gonna go back in the hut.

No matter how bad the weather, we always put up our trip-wires.

Just in case we get a visitor in the night, when we're asleep.

We put up trip wires, which are basically explosion fences.

A bear comes along, trips a wire and an explosion goes off,

which will hopefully scare the bear off, or at least wake us up.

Two bottles! One Bacardi, one Cointreau found in the hut...

However, both unmistakably paraffin.

Well, we may get driven to them!

In Antarctica, the Italian scientific research

involved diving under the ice

to measure light coming from the surface to analyse algal growth.

This gave Doug the opportunity to get some shots below the ice.

A dubious pleasure.

It's quite nice, really big platelets of ice, like this.

And very impressive when Mac was crawling in amongst it

trying to find a space for the light sensor.

Very nice. Browns, greens, all kind of colours.

Even a Weddell seal came by briefly

and swam away. Good. Nice.

The main problem under the ice is NOT the cold.

The water has quite a constant temperature, just above freezing -

far warmer than at the surface.

The real danger of diving under the ice is losing your way out.

One of the advantages of working in the polar regions

is that in the summer, the sun never sets.

You can work out on the ice round the clock.

Just as well for Doug and Jason,

who still haven't found their polar bear.

Freeze-dried chicken in curry.

Absolutely lovely at minus 20, nice and warm.

Mix it with a bit of water.

Despite the food, things did seem to be looking up.

At long last, after days of searching,

they had found a polar bear cub...

..and its mother.

The light was ideal for filming,

but the bears were not being co-operative.

That was so frustrating.

We had this female and cub, we've been kind of watching her,

doing a bit of hunting and never very close.

She came into this nice position, then I took a few steps towards her.

She was a long way away, but she completely reacted the wrong way,

and I lost all her confidence, and now she's off somewhere

and it's such a nice day for filming.

I thought we had it, and, oh...

Despite the setbacks, two days later, Doug's persistence paid off.

He was able to win back the confidence of the bears.

It's five o'clock in the morning, and the 18th day of the shoot.

I think we just cracked it. We had a female there,

with the cub. She did a lot of pouncing about.

50, 80 metres away. A nice sort of distance.

You know what she did when she was finished?

She was so relaxed, she sat down,

and gathered the cub into her and suckled her head onto the camera.

I tell you, it's a weight off your mind

when you get something like that. It's incredible.