The sun is absent for up to half the year in the polar regions.
When it returns, at the beginning of spring,
its warmth will transform this magical ice world.
The greatest seasonal change on our planet is now underway.
Antarctica is still locked in ice, and surrounded by a frozen ocean.
Nonetheless, there are signs of spring.
Adelie penguins are arriving - just the males.
They've spent five months at sea, where it's warmer than it is on land
and now they're in a hurry, for spring will be short.
They have travelled 6,000 miles across the ocean
since leaving their colony last year,
and now they're returning to breed.
They cannot lay their eggs on ice, for they would freeze,
so they have to come here, where there is bare rock.
Over the coming months,
the few parts of Antarctica that are ice-free
will be the stage on which five million Adelies
will build their nests.
To construct one, they need pebbles, and without a good-looking nest,
a male will be unable to attract a female,
when they at last arrive.
An impressive property demonstrates your worth as a mate.
It takes stones of all shapes and sizes to build a decent nest,
and finding ones that are just right is not easy.
So some penguins turn to a life of crime.
The one who has been robbed seems unaware
that the thief is just over his shoulder, and looking for more.
The thief's nest is coming along nicely,
probably because he keeps a particularly sharp lookout
for robbers -
after all, it takes one to know one.
It's still cold, but the early season sun
does lift the temperature by a few degrees.
That, however, can have unexpected, even dangerous consequences.
The sea is heating faster than the land,
pulling cold air from the middle of the continent towards the coast.
These katabatic winds are stronger than any hurricane.
They are the coldest and the most ferocious on the planet.
The storms catch many new arrivals by surprise,
and are the reason that spring here is, in fact, the deadliest season.
Here, early birds take a great risk.
Some years, entire colonies are lost,
buried beneath the snow.
The survivors of this storm
must hope that the females prove to be worth the wait
when they finally decide to turn up.
Spring in the north of our planet.
The sun, after an absence of six months,
breaks the horizon for the first time.
A female polar bear emerges from her den beneath the snow.
The sun must be a welcome relief after so long in the darkness.
Her den is on a high slope,
well away from hungry male bears who would kill her cubs,
but close enough to the sea ice where she can find food
for her extraordinary new family...
..three young cubs.
If she can raise them all to independence,
it'll be a rare achievement.
One of the cubs is underweight,
and will be fortunate to survive these early weeks.
If the family reaches the sea ice, where the female can catch seals,
her milk will be enriched,
and the smaller cub will quickly gain size and strength.
The sea ice, though, is a dangerous place.
The male polar bear has been out on the frozen ocean all winter.
Times have been lean,
and a bear cub would certainly make a welcome snack.
However, the mainstay of his diet is seals,
and now is the time when they have their pups.
The surface of the frozen sea is marked with pressure ridges,
and cracks created by the fluctuating tides.
Both are good places to look for seals.
He has detected a seal den beneath the ice.
Now he must pin-point it, using only his extraordinary sense of smell.
By treading lightly, he can avoid scaring his prey.
He will need to punch through a metre of snow to reach the den,
and if his aim is not exactly on target,
the seal will certainly escape.
In fact, the ringed seal abandoned her den
just a few hours ago
and her pup has climbed up onto the surface.
It's more exposed up here, but it's easier to see an approaching bear.
The pup is well camouflaged.
Difficult to see when its mother has left it to go fishing.
But he can still smell it.
Nine out of ten polar bear hunts end in failure.
The sun's warming effect on the Arctic is increasing
and the sea ice is showing the first signs of weakening.
Inland, the northern rivers are still locked in ice.
The frozen waterfalls are like dams
holding back billions of tonnes of fresh water
that has not moved for almost six months.
The vast water shed lies motionless,
but as spring advances, it beings to stir.
The frozen waterfalls start to weaken.
Above them, the pressure is mounting.
Now, from high above, whole sections can be seen to be on the move.
The waterfalls are straining to hold back the force
that is building up above them.
The dam bursts...
and the river is unleashed.
Millions of tonnes of ice grind their way downstream,
driven by the unstoppable force of the meltwater.
Boulders and trees are plucked from the bank side.
Within just a few days, the rivers of the north are all running.
The Arctic's fresh water is flowing again.
These vast floods contain 10% of the world's fresh water
and as they enter the ocean, they accelerate the break-up.
Soon, an area of sea ice the size of Australia
will vanish from the Arctic Ocean.
As the huge ice-sheet breaks up, wildlife returns to the North.
The polar bear mother has made it to the edge of the sea-ice,
but the smallest cub is nowhere to be seen.
It's a sad outcome,
but the disappearance improves the chances of the remaining two,
who now have more milk to share.
Having led her cubs to the edge of the ice,
the mother's next challenge is to catch a seal.
Not easy with these two in tow!
CUB GRUNTS
Her prey beneath the ice can detect the slightest vibration,
especially from bears.
And this is not the stealthiest of hunting parties.
BEAR GROWLS
Leaving her cubs behind however, is not an option -
a male bear would eat one in a moment.
This is not going to be easy.
It seems that the cubs already see themselves as fearsome hunters,
but they're still young enough to accept their mother's discipline.
It's the naughty corner for you!
This is not going to be their lucky morning.
They saw no sign of a seal and the cubs are hungry again.
The mother has been nursing for four months without once eating herself,
and now her milk is drying up.
She must catch a seal soon, or the whole family will starve.
The challenge of finding food is getting harder.
She needs the sea-ice as a platform from which to hunt
and it's breaking up faster with each passing day.
Even the ice around the coast is starting to break.
The first cracks here are eagerly awaited by some...
..narwhals.
The unicorns of the North are on a mission -
to reach the new fishing grounds
in the bays that have been frozen-up all winter, but are now opening up.
To get to them, the narwhals must travel down leads -
temporary cracks in the ice.
But these new roads could close at any time,
cutting off the air that they need to breathe.
The road narrows until there's barely room for one-way traffic.
Then, a surprise...
narwhals coming from the other direction.
It's a stand-off.
Each team faces an armoury of sharp tusks.
Finally, one side concedes
and everyone continues in the same direction.
Within a week, the remaining bays break wide open,
and the narwhals turn their attention to social matters.
No-one knows exactly what the narwhal's tusk is for.
Some say it's used for fencing,
yet these exchanges are too gentle to be real fights.
Perhaps these encounters
are to remind each animal of where it stands in Narwhal society.
We may never know.
Bizarre creatures appear as if from nowhere.
The tiny plankton they eat have been fuelled by the increasing warmth
of the sun and fertilised by the nutrients
brought down by the great rivers and released from the melting sea-ice.
Sea gooseberries strain the water with their stinging filaments,
their beating cilia scatter the sun's rays
into dazzling bursts of colour.
Beneath the breaking sea ice, a predatory sea slug
flies through the water on translucent wings.
It's on the trail of a peculiar swimming snail.
And now the most voracious of the plankton-eaters
swim up from the depths...
Arctic cod.
SEAGULLS CRY
The shoals can be enormous, some containing 500 million fish
and predators travel hundreds of miles to feast on them.
The cod harvest is THE annual event
for the birds and seals of the far North.
For just a short period,
the combination of the strengthening sun, the newly-flowing rivers
and the breaking sea ice make the Arctic Ocean teem with life.
The land is also transformed by the sun's heat.
The small patches of bare ground that appear are darker than the snow
so they absorb more of the sun's energy.
This accelerates the melt.
The Arctic tundra is unveiled.
By tracking the sun, Arctic poppies catch its rays around the clock,
so their flowers are always warmer than their surroundings.
For early season insects, this warmth is even more valuable
than nectar if they're to stay active in the cold.
The woolly bear caterpillar does not need the warmth from flowers
to kick-start its spring.
It's always the first insect to appear after the snow retreats
and the story of how it does so is truly astonishing.
At the start of spring, the caterpillar eats as fast as it can,
as indeed it must, for this far north, the season will be brief.
The days shorten only too soon, but the caterpillar has not yet
got enough reserves to transform itself into a moth.
It can't leave the Arctic, for it can't fly,
so it settles down beneath a rock.
The sun's warmth rapidly dwindles.
Beneath the rock, the caterpillar is out of the wind,
but the cold penetrates deep into the ground.
Soon, its heart stops beating.
It ceases to breathe, and its body starts to freeze -
first its gut, then its blood.
Spring.
After four months of darkness, the Arctic begins to thaw.
And the caterpillar rises from the dead.
By the time the first shoots of willow appear
in the early spring, the woolly bear is already eating.
But no matter how fast the woolly bear eats, it will not have time
to gather enough food this year, either,
and the cold closes in once again.
Year after year, the caterpillar slows down in the autumn
and then freezes solid.
But eventually, a very special spring arrives.
This one will be its last.
It's now 14 years old - the world's oldest caterpillar.
Its remaining days now become frantic.
It starts to weave a silk cocoon.
Inside, its body is changing into one that can fly and search,
abilities that will be crucial in the days ahead.
It's waited over a decade for this spring and now, its time is near.
All across the Arctic, moths are emerging.
After completing their 14-year preparation,
they now have just a few days to find a partner and mate.
No life illustrates more vividly the shortness of the Arctic spring
or the struggle to survive in this most seasonal of places.
As spring advances,
the transformation of the tundra continues.
THEY SQUEAK
CHEEPING
Migrants begin arriving from the south
and suddenly the tundra is alive with birds and chicks.
The Arctic's transformation is complete.
This influx of life is good news for some permanent residents.
Food is rarely plentiful out on the tundra.
The Arctic wolves must make the most of this boom while it lasts.
HONKING
The wolves must gather as much food as they can.
Many miles from here, other members of the pack are relying on them.
HOWLING
This barren landscape is a hard place to make a living,
forcing wolf packs to be smaller here than further south.
Six hungry mouths to feed.
The cubs are just over a month old.
BARKING AND YELPING
The ducks are devoured instantly, but on the long journey home,
they also caught an Arctic hare, a mainstay of the tundra diet,
and one the cubs seem to be particularly keen on.
Uneaten food is usually hidden for leaner times,
but there will be no leftovers today.
The cubs are growing fast and are always hungry.
GROWLING
The good times are certainly back, but these white wolves remind us
of the Arctic's less welcoming side.
Their coats are pale to conceal them during the long, snowy winter.
It's easy to forget that one month ago,
this land was a barren, white desert.
At the southern end of our planet,
the Antarctic sea ice is still at its greatest extent.
But there are a few islands on its outer edge
that the sea ice never quite reaches.
South Georgia is washed by the rich waters of the Southern Atlantic
and the comparative warmth of the sea
takes the edge off the vicious southern winter.
It's even possible for a few hardy animals,
like the Wandering Albatross, to live here throughout the whole year.
The enormous Albatross chicks take 13 months to fledge,
so they have no choice but to sit here throughout the winter.
It can't be easy, but the thick layer of fluffy down
keeps out the worst of the cold.
Their parents travel thousands of miles to collect the fish
and squid they need to stay warm and to grow.
The season is turning and storms blow in with little warning.
King penguins have also been here all winter.
Their chicks survive by huddling in creches to conserve their heat.
A solitary bird here standing alone would quickly die of exposure.
It seems for a hardy few, violent storms are a price worth paying
for year-round fishing in the rich waters of the southern ocean.
The penguins have had the beach to themselves all winter,
but that is a luxury that will not last.
As the winter storms subside, life begins to return.
For half of the year, South Georgia has the greatest concentration
of sea birds in the world
and most of them arrive in the early spring.
Macaroni penguins make the most impressive entrance -
over five million pairs of them.
They are the world's most numerous penguin
and half of them are now here.
The arrival is complete.
Courtship is next on South Georgia's busy spring schedule.
The Wandering Albatross has the most elaborate display.
MIXED-PITCH "POPPING"
These two are renewing their bonds after being months apart.
Wanderers pair up in their teens
and can spend a further 50 years together -
one of the longest partnerships in the animal world.
By the middle of spring, the snows have cleared
from the coves and the low ground.
The beaches are almost free of ice too,
but that isn't the biggest change facing the king penguins.
Their peaceful waterfront has turned into an obstacle course of blubber.
The elephant seals have arrived.
This beach now contains a greater mass of animals
than any other in the world.
The young seals were conceived here a year ago,
and now that they've been born,
their mothers are ready to mate again.
The mating rights on this patch of the beach belong to a beach master.
His harem contains 50 females -
females that are coveted by others.
His authority is being challenged.
This rival means business.
This could be the beach master's first serious test
of his spring campaign.
The beach master himself weighs four tonnes,
but this rival is his equal.
When these titans clash,
bones crunch.
He has won the first battle,
but he may have to defend his harem every hour for the next month.
If he can stay master of his beach for this period,
many of the young born here next year will be his.
It's the end of spring on the Wandering Albatross's cliffs,
their season for fledging.
Last year's chicks have lost their fluffy down
and step up to the challenge of getting into the air.
SQUAWKING
An albatross is not very competent on the ground,
so until it can fly, it isn't good for much
and this makes the maiden flight
THE crucial event in an albatross's life.
Managing the largest wing span in the world takes practice...
..lots of it.
Weeks can go by like this.
Certainly the winds must be right, but it does appear that for some,
the problem is something of a mental one.
At last.
The Southern Ocean beckons.
This bird's feet will not touch land again for five years.
1,000 miles further south,
on the edge of the Antarctic continent,
the sea ice is only just starting to break.
But the Adelie penguin's activities are certainly warming up.
The males have now finished their nests by fair means or foul
and the females are finally returning,
just as the weather is improving.
Now their courtship can begin.
RHYTHMIC CAWING
The eggs are laid and the females leave the job of incubating them
to the males, while they go fishing out on the fragmenting sea ice.
Killer whales.
It's teamwork that makes killer whales so dangerous.
And THIS is a big team.
There is no real need for the penguins to be alarmed.
These killer whales are a kind that only eats fish.
Rising out is simply the best way for the whales to work out
which cracks lead towards the coast and better fishing.
A new generation of Adelies steps forth
into the short Antarctic spring
to be nurtured by industrious parents who've taken great risks
to give their young a head start.
They will need to grow fast if they are to fledge
and leave before the freeze sets in again.
It's a battle they will win or lose over the approaching summer.
Next time,
summer and the sun never sets in the polar regions.
The heat is on for the Frozen Planet.
To film the entire breeding cycle of the Adelie penguin,
Frozen Planet sent a team to one of the world's largest colonies,
at Cape Crozier, Antarctica.
I've heard so much about it and read so much about it.
Finally we're going to get there. It's good.
Cameraman Mark Smith and director Jeff Wilson
plan to spend the next four months living amongst the penguins
in a location first visited by the early explorers a century ago.
So arduous was Scott's winter expedition to Cape Crozier
that it became known as "the worst journey in the world".
Wow, it's fantastic!
Modern means make Mark and Jeff's journey a more comfortable affair,
but once there, they will be tested
to the very limits of their endurance.
The pair arrive in early spring
with enough supplies to survive the next four months working alone in the Antarctic wilderness.
We're here!
Scott's legend of Cape Crozier tells of some extreme weather,
to say the least,
so Mark and Jeff take advantage of the clear conditions,
in the knowledge that the Adelies' arrival is imminent.
But the next morning, things take a turn for the worse.
We've just come up to this ridge
to go and check what it looks like down in the colony.
ROARING
But even here you can hear...
a huge kind of roaring noise
up on the hill. Never really heard anything like that before.
Up there, it must be blowing the most almighty gale,
and that is, pff, you know, just a mile away or something.
So that means that that could get here very, very quickly.
So yeah, it makes you slightly scared. It's weird, isn't it?
The winds here are famously ferocious,
and with so little experience of this location,
Mark and Jeff retreat to the relative shelter of their hut.
I was just stood here thinking, "Well, it can't be too bad,
"because we haven't seen rocks starting to blow around yet,"
and just at that moment a rock took off and rolled down there.
Oh, dear, it's getting stronger.
By the second day of the storm, the winds reach 80 miles an hour,
and it's apparent that even getting lunch from the outside larder
is too risky.
To their increasing alarm, the storm continues to build.
All afternoon, it's been blowing about...must have been 100 miles an hour,
and in the last half-hour it's just got a lot stronger.
Aside from being utterly terrified,
there's the added worry that our gear is stashed outside somewhere.
We didn't have room for it in the hut,
and we don't know whether it's going to be there in the morning,
which could spell the end of our trip.
On the third day of the storm, the winds hit 130 miles an hour.
The hut starts to shake from its very foundations,
and Mark and Jeff's situation becomes critical.
The wind's so strong,
it's constantly blowing the pilot light out on the paraffin stove,
so the temperature's dropping.
The wind's rising. Sounds like the bloody roof's coming off.
The really scary thing is that had we gone out down to the colony
and tried to film today,
there's a very high likelihood that we'd be dead by now.
And I don't say that lightly.
There's no way we would have seen this through down there.
And that is quite sobering.
After four terrifying days inside the hut,
the winds finally drop, and Mark and Jeff are keen to see
what, if any, equipment has survived.
All the real important stuff, the camera stuff, is all still here,
and it's still strapped to this rock.
You have no idea how much joy that gives us.
We can get on with our jobs now!
The first things to welcome us to the colony are these skuas,
which come in and batter us from above.
At its height, the colony will swell to over half a million penguins,
and in the 24-hour daylight of the Antarctic summer,
Mark and Jeff spend all of their waking hours filming.
Unpredictable weather continues to force the team
to climb the two miles back to the relative shelter of their hut,
and Mark devises a novel way of testing the wind speed.
Stop!
After six weeks, the first sign that the pair
might be tiring of their penguin neighbours.
Over there's a leopard seal.
It's the first non-penguin-looking animal in two months!
Will you look at that? It's a leopard seal!
I was down here filming the penguins coming in,
followed this penguin that came out of the waves up the beach,
very nice shot, stopped the shot
and there in the middle of the frame was this completely white penguin...
trying to get into the shot.
We suspect that he might be following us around now.
Working around the clock for this length of time
in the presence of half a million screaming penguins
would test anyone's resolve.
I hate carrying gear. Hear that? I hate carrying gear. I don't want to do it any more.
Midway through their trip, after more than 1,000 hours amongst the penguins,
their grip on reality is beginning to loosen.
Here we are on the penguin superhighway,
where the penguins go down to the sea.
It seems that they follow the American system...
driving on the right, going down to the sea on the right,
coming back from the sea on the left as you're facing the sea.
I'm fairly sure that yesterday they were doing the British system.
The legendary Cape Crozier weather soon snaps them back into reality.
I'm just filming the penguins
with this huge kind of wind storm
coming over the ice cap,
the kind of thing we were warned about by the guy who was here before,
who has been here several years, saying "With skies like this, you should run for home".
But we're just going to stay and film it.
Because we're the BBC!
But in this part of the world,
working for the BBC doesn't count for much.
Within minutes, the winds reach hurricane strength,
and the crew are in serious trouble.
So, we've got to now venture out and go about a mile up this valley,
which looks like it's got about 80mph winds blowing down it.
So it's going to be quite an adventure.
Bit frightening, though, really.
After three months, the pair are now fully aware
of the strength the winds can reach. Getting a little stronger now!
There is a very real danger
that they could be separated and lost in these white-out conditions.
We're going to go BLEEP.
Two terrifying hours later, and their relief
at finally reaching the hut is tangible.
THEY LAUGH
With a month still to go at Cape Crozier,
Mark, Jeff and the penguins will face many more storms like this.
But it seems their greatest challenge
will be to maintain their sanity.
Here we are, travelling through the Antarctic by sled.
We're being pulled by a herd of huskies. Oh!
12 of them panting out front, breath steaming from their mouths.
And as we go along,
we see the happy people waving at us.
For a free Open University illustrated poster of polar regions,
call the number on the screen,
or order online by going to bbc.co.uk/frozenplanet
and follow the links to Open University.
Subtitles by Red Bee Media Ltd
E-mail subtitling@bbc.co.uk