0:00:29 > 0:00:35The polar regions are the coldest, most extreme places on our planet.
0:00:36 > 0:00:39Winds reach 200 miles an hour
0:00:39 > 0:00:43and temperatures drop as low as minus 70 degrees centigrade.
0:00:47 > 0:00:49Only the toughest can survive here.
0:00:54 > 0:00:57Over the course of a year, polar animals must overcome
0:00:57 > 0:01:04not just the cold, but the most extreme seasonal changes on Earth.
0:01:11 > 0:01:18In the High Arctic, winter is one long night that lasts for months.
0:01:20 > 0:01:25It's also the time when new lives begin.
0:01:39 > 0:01:43Polar bear cubs are born blind and tiny.
0:01:45 > 0:01:50An early birth is easier on the mother, who is barely awake.
0:02:09 > 0:02:12Despite her sleepiness, her instinct
0:02:12 > 0:02:14to nurse is overwhelming.
0:02:26 > 0:02:29CUBS SQUEAK
0:02:29 > 0:02:31The cubs' clucking calls
0:02:31 > 0:02:34stimulate her to produce milk.
0:02:34 > 0:02:35And what milk!
0:02:35 > 0:02:38It's nine times richer than our own,
0:02:38 > 0:02:39and enables her to double
0:02:39 > 0:02:41their weight every few weeks.
0:03:04 > 0:03:10The sun is returning after an absence of nearly four months.
0:03:10 > 0:03:14And its warmth starts to transform this magical ice world.
0:03:17 > 0:03:18Spring has arrived.
0:03:33 > 0:03:38The greatest seasonal change on our planet is now under way.
0:03:45 > 0:03:48Antarctica is still locked in ice,
0:03:48 > 0:03:51and surrounded by a frozen ocean.
0:03:52 > 0:03:54Nonetheless, there are signs of spring.
0:04:02 > 0:04:06Adelie penguins are arriving. Just the males.
0:04:06 > 0:04:08They've spent five months at sea,
0:04:08 > 0:04:10where it's warmer than it is on land,
0:04:10 > 0:04:12and now they're in a hurry,
0:04:12 > 0:04:15for spring will be short.
0:04:22 > 0:04:25They have travelled 6,000 miles across the ocean
0:04:25 > 0:04:30since leaving their colony last year and now, they're returning to breed.
0:04:30 > 0:04:33They cannot lay their eggs on ice for they would freeze.
0:04:33 > 0:04:37So they have to come here, where there is bare rock.
0:04:41 > 0:04:43Over the coming months,
0:04:43 > 0:04:47the few parts of Antarctica that are ice-free will be the stage
0:04:47 > 0:04:51on which five million Adelies will build their nests.
0:04:57 > 0:05:00To construct one, they need pebbles.
0:05:00 > 0:05:02And without a good-looking nest,
0:05:02 > 0:05:06a male will be unable to attract a female.
0:05:06 > 0:05:08When they at last arrive.
0:05:08 > 0:05:12An impressive property demonstrates your worth as a mate.
0:05:15 > 0:05:19It takes stones of all shapes and sizes to build a decent nest
0:05:19 > 0:05:23and finding ones that are just right is not easy.
0:05:28 > 0:05:32So, some penguins turn to a life of crime.
0:06:10 > 0:06:13The one who has been robbed seems unaware
0:06:13 > 0:06:17that the thief is just over his shoulder and looking for more.
0:06:27 > 0:06:31The thief's nest is coming along nicely, probably because
0:06:31 > 0:06:35he keeps a particularly sharp lookout for robbers.
0:06:35 > 0:06:38After all, it takes one to know one.
0:06:46 > 0:06:50By fair means or foul, the males must finish their nests quickly
0:06:50 > 0:06:54if they're to raise a family over the short Antarctic summer.
0:07:08 > 0:07:10In the north of our planet,
0:07:10 > 0:07:15the weak sunshine is a welcome relief after the months of darkness.
0:07:20 > 0:07:26Finally, it's time for polar bear families to emerge from their dens.
0:07:31 > 0:07:34This is not the easiest place
0:07:34 > 0:07:36to take your first steps.
0:07:36 > 0:07:37And the little ones will need plenty
0:07:37 > 0:07:40of encouragement from their mother.
0:07:48 > 0:07:50If she can raise all three to independence,
0:07:50 > 0:07:53it will be a rare achievement.
0:08:07 > 0:08:11During the months underground, all they've known is their mother.
0:08:11 > 0:08:15Now, the big, wide world holds all sorts of challenges
0:08:15 > 0:08:18for these brave, young explorers.
0:08:26 > 0:08:31The mother leads her new family out over the frozen sea
0:08:31 > 0:08:32to hunt for seals.
0:08:37 > 0:08:40But the spring melt is already under way.
0:08:44 > 0:08:48All across the Arctic, the sun's warming effect is increasing.
0:08:48 > 0:08:52Sea ice is showing the first signs of weakening.
0:08:55 > 0:08:59Inland, the northern rivers are still locked in ice.
0:09:03 > 0:09:06The frozen waterfalls are like dams, holding back billions of tonnes
0:09:06 > 0:09:11of fresh water that has not moved for almost six months.
0:09:11 > 0:09:14CREAKING
0:09:16 > 0:09:17The vast watershed lies motionless.
0:09:17 > 0:09:19But as spring advances,
0:09:19 > 0:09:22it begins to stir.
0:09:27 > 0:09:29The frozen waterfalls start to weaken.
0:09:29 > 0:09:32Above them, the pressure is mounting.
0:09:38 > 0:09:42Now, from high above, whole sections can be seen to be on the move.
0:09:51 > 0:09:54The waterfalls are straining to hold back the force
0:09:54 > 0:09:56that is building up above them.
0:10:04 > 0:10:06The dam bursts...
0:10:07 > 0:10:09..and the river is unleashed.
0:10:35 > 0:10:37These vast floods accelerate the break-up.
0:10:39 > 0:10:43Soon, an area of sea ice the size of Australia
0:10:43 > 0:10:45will vanish from the Arctic Ocean.
0:10:54 > 0:10:57The spring melt breaks the ice for visitors.
0:11:00 > 0:11:01Narwhals.
0:11:06 > 0:11:08The unicorns of the North are on a mission -
0:11:08 > 0:11:11to reach the new fishing grounds
0:11:11 > 0:11:15in the bays that have been frozen up all winter, but are now opening up.
0:11:23 > 0:11:27To get to them, the narwhals must travel down leads,
0:11:27 > 0:11:29temporary cracks in the ice.
0:11:30 > 0:11:33But these new roads could close at any time,
0:11:33 > 0:11:36cutting off the air that they need to breathe.
0:11:39 > 0:11:43The road narrows, until there's barely room for one-way traffic.
0:11:45 > 0:11:49Then, a surprise. Narwhals coming from the other direction.
0:11:54 > 0:12:00It's a stand-off. Each team faces an armoury of sharp tusks.
0:12:18 > 0:12:21Finally, one side concedes.
0:12:21 > 0:12:25And everyone continues in the same direction.
0:12:31 > 0:12:35The melting sea ice reveals open ocean.
0:12:35 > 0:12:38And the changes on land are no less dramatic.
0:12:44 > 0:12:47Beneath the snow, life is already stirring.
0:12:49 > 0:12:53The Arctic tundra is unveiled.
0:13:00 > 0:13:02Water that has been locked in ice all winter
0:13:02 > 0:13:05once again flows freely.
0:13:28 > 0:13:31A landscape that was only white now bursts with colour.
0:13:50 > 0:13:53Migrants begin arriving from the south
0:13:53 > 0:13:57and, suddenly, the tundra is alive with new families.
0:14:04 > 0:14:08These Arctic wolf cubs are just over a month old
0:14:08 > 0:14:11and always looking out for their next meal.
0:14:21 > 0:14:23Their parents have been hunting.
0:14:23 > 0:14:28Their offerings are devoured instantly.
0:14:29 > 0:14:33Arctic hare is a mainstay of the tundra diet
0:14:33 > 0:14:36and one the cubs seem particularly keen on.
0:14:40 > 0:14:43Uneaten food is usually hidden for leaner times,
0:14:43 > 0:14:46but there will be no leftovers today.
0:14:46 > 0:14:50The cubs are growing fast and are always hungry.
0:15:01 > 0:15:05The good times are certainly back, but these white wolves remind us
0:15:05 > 0:15:09of the Arctic's less welcoming side.
0:15:10 > 0:15:12Their coats are pale to conceal them
0:15:12 > 0:15:14during the long, snowy winter.
0:15:16 > 0:15:19It's easy to forget that one month ago,
0:15:19 > 0:15:22this land was a barren, white desert.
0:15:22 > 0:15:27But finding a decent meal here is never easy.
0:15:31 > 0:15:34The parents travel up to 80 miles in a day in search of
0:15:34 > 0:15:37more substantial prey for their growing family.
0:15:50 > 0:15:53Musk oxen are immensely powerful.
0:15:53 > 0:15:56And their sharp horns can kill.
0:15:56 > 0:15:59But a calf is more vulnerable.
0:16:22 > 0:16:25The two wolves work together to split the herd
0:16:25 > 0:16:26and isolate their victim.
0:16:47 > 0:16:51It seems that the wolf cubs will, at last, eat well.
0:16:56 > 0:16:57But the herd regroups.
0:17:06 > 0:17:09The cattle ride to the rescue.
0:17:22 > 0:17:25The wolves cannot penetrate the wall of horns.
0:17:28 > 0:17:30The herd are protecting their calf.
0:17:33 > 0:17:36For the musk oxen, it's all for one
0:17:36 > 0:17:38and one for all.
0:17:44 > 0:17:47It's a struggle for all polar animals to feed themselves
0:17:47 > 0:17:51and their families in the brief Arctic summer.
0:17:53 > 0:17:57But there is one tiny creature of the tundra that has found
0:17:57 > 0:18:02an astounding solution to the shortness of the season.
0:18:02 > 0:18:04The woolly bear caterpillar.
0:18:05 > 0:18:10It's always the first insect to appear after the snow's retreat.
0:18:12 > 0:18:15The caterpillar then eats as fast as it can and, indeed it must,
0:18:15 > 0:18:19for this far north, summer only lasts a few weeks.
0:18:27 > 0:18:32The days shorten only too soon, but the caterpillar has not yet
0:18:32 > 0:18:36got enough reserves to transform itself into a moth.
0:18:36 > 0:18:40It can't leave the Arctic, for it can't fly.
0:18:40 > 0:18:42So, it settles down beneath a rock.
0:18:43 > 0:18:46The sun's warmth rapidly dwindles.
0:18:52 > 0:18:55Beneath the rock, the caterpillar is out of the wind,
0:18:55 > 0:18:58but the cold penetrates deep into the ground.
0:19:01 > 0:19:03Soon, its heart stops beating.
0:19:03 > 0:19:07It ceases to breathe and its body starts to freeze.
0:19:08 > 0:19:11First, its gut. Then, its blood.
0:19:35 > 0:19:38After four months of darkness,
0:19:38 > 0:19:40the Arctic begins to thaw.
0:19:45 > 0:19:48And the caterpillar rises from the dead.
0:19:55 > 0:19:58By the time the first shoots of willow appear in the early spring,
0:19:58 > 0:20:00the woolly bear is already eating.
0:20:02 > 0:20:05But no matter how fast the woolly bear eats,
0:20:05 > 0:20:09it will not have time to gather enough food this year either,
0:20:09 > 0:20:12and the cold closes in once again.
0:20:31 > 0:20:35Year after year, the caterpillar slows down in the autumn
0:20:35 > 0:20:37and then freezes solid.
0:20:46 > 0:20:49But, eventually, a very special summer arrives.
0:20:54 > 0:20:56This one will be its last.
0:21:01 > 0:21:03It's now 14 years old.
0:21:03 > 0:21:07The world's oldest caterpillar.
0:21:07 > 0:21:10Its remaining days now become frantic.
0:21:10 > 0:21:12It starts to weave a silk cocoon.
0:21:16 > 0:21:18Inside, its body is changing
0:21:18 > 0:21:21into one that can fly and search.
0:21:21 > 0:21:25Abilities that will be crucial in the days ahead.
0:21:38 > 0:21:42All across the Arctic, moths are emerging.
0:21:42 > 0:21:46After completing their 14-year preparation,
0:21:46 > 0:21:49they now have just a few days to find a partner and mate.
0:21:55 > 0:21:58No life illustrates more vividly the struggle to survive
0:21:58 > 0:22:01in this most seasonal of places.
0:22:07 > 0:22:11In a matter of weeks, the north will be frozen over once again.
0:22:14 > 0:22:16At the southern end of our planet,
0:22:16 > 0:22:19Antarctica is still surrounded by sea ice.
0:22:29 > 0:22:34The Antarctic spring arrives first at the outer islands.
0:22:36 > 0:22:39Although the beaches of South Georgia are now ice-free,
0:22:39 > 0:22:42the King Penguins face a new challenge.
0:22:42 > 0:22:48Their peaceful waterfront has turned into an obstacle course of blubber.
0:22:50 > 0:22:52The elephant seals have arrived.
0:22:56 > 0:22:59The beachmaster's authority is being challenged.
0:23:05 > 0:23:07This rival means business.
0:23:09 > 0:23:13The beachmaster owns the females here and must to fight keep them.
0:23:26 > 0:23:33The beachmaster himself weighs four tonnes, but this rival is his equal.
0:24:30 > 0:24:33The beachmaster has won the first battle,
0:24:33 > 0:24:38but he may have to defend his harem every hour for the next month.
0:24:38 > 0:24:41If he can stay master of his beach for this period,
0:24:41 > 0:24:44many of the young born here next year will be his.
0:25:01 > 0:25:04In summer, the Southern Ocean bursts with life.
0:25:33 > 0:25:38No bird is more at home in water, and they are masterful surfers.
0:25:43 > 0:25:48Penguins are found only in the Southern Hemisphere.
0:25:50 > 0:25:55They can't fly, but they don't need to. There are no polar bears here.
0:26:03 > 0:26:06These are Gentoo Penguins.
0:26:06 > 0:26:11Each spring, they come ashore to lay their eggs and rear their young.
0:26:11 > 0:26:12Their hungry chicks demand
0:26:12 > 0:26:15so much seafood that both parents have to go fishing.
0:26:22 > 0:26:24And fishing can be dangerous.
0:26:40 > 0:26:42A southern sea lion.
0:26:46 > 0:26:51It uses the speed of a breaking wave to catch up with the Gentoos.
0:27:06 > 0:27:08Sea lions normally eat fish,
0:27:08 > 0:27:12so he's used to catching streamline swimmers.
0:27:12 > 0:27:15But the Gentoos seem more than his match out at sea.
0:27:15 > 0:27:17He must change tactics.
0:27:21 > 0:27:25Perhaps it would be easier in the shallows.
0:27:48 > 0:27:51But, no, it seems penguins are uncatchable in water.
0:27:57 > 0:27:59How about on land?
0:28:00 > 0:28:04The penguin's wings, so powerful for swimming,
0:28:04 > 0:28:06are of no help when it comes to running.
0:28:06 > 0:28:09Now, surely, the sea lion has a chance?
0:28:14 > 0:28:18But on the beach, both are like fish out of water.
0:28:40 > 0:28:45Rarely do hunter and hunted play their roles with so little skill.
0:28:45 > 0:28:47The outcome is anyone's guess.
0:29:17 > 0:29:20Every summer, over 40 million penguins
0:29:20 > 0:29:23take to the Southern Ocean to feed.
0:29:29 > 0:29:31They're joined by thousands of whales.
0:29:31 > 0:29:33Minkes are the most numerous.
0:29:40 > 0:29:44They all come here to harvest the richest ocean on Earth.
0:29:50 > 0:29:55Southern humpbacks have travelled 4,000 miles from the equator
0:29:55 > 0:29:58to get here in time for the Antarctic summer.
0:30:03 > 0:30:07Each year, the sea ice that surrounds Antarctica melts,
0:30:07 > 0:30:10effectively halving the size of the continent
0:30:10 > 0:30:12and exposing rich waters around its coastline.
0:30:15 > 0:30:19It's the most spectacular seasonal change occurring
0:30:19 > 0:30:21anywhere on our planet.
0:30:24 > 0:30:27The remnants of the sea ice are occupied by sunbathing seals
0:30:27 > 0:30:30that have been here all winter.
0:30:33 > 0:30:37But new arrivals are following the retreating ice edge.
0:30:38 > 0:30:41And they have come here to hunt.
0:30:48 > 0:30:50Killer whales. The ocean's top predator.
0:30:52 > 0:30:54Killers are like wolves,
0:30:54 > 0:30:57for they will hunt animals far larger than themselves.
0:30:57 > 0:30:59But even smaller prey are a problem
0:30:59 > 0:31:01if you can't reach them.
0:31:08 > 0:31:10The solution is teamwork.
0:31:12 > 0:31:15Swimming in perfect formation, they flick their tails in unison
0:31:15 > 0:31:18and create a wave that cracks the ice.
0:31:27 > 0:31:30They regroup and assess the damage.
0:31:30 > 0:31:32A more powerful wave is needed.
0:31:37 > 0:31:40The ice floe is breaking up.
0:31:42 > 0:31:45Now, they are close enough to get a good look at their target.
0:31:45 > 0:31:50The seal is a crabeater, sharp-toothed and feisty.
0:31:50 > 0:31:51Not their favourite.
0:31:55 > 0:31:57The wolves of the sea move on
0:31:57 > 0:31:59in search of easier quarry.
0:32:04 > 0:32:06A Weddell seal, that's better.
0:32:07 > 0:32:11These are more docile and easier to tackle.
0:32:14 > 0:32:17The pod stays close together and travels silently.
0:32:19 > 0:32:23This time, they unleash a far more powerful wave,
0:32:23 > 0:32:25and with astonishing accuracy.
0:32:32 > 0:32:35These big waves are not intended to break the ice,
0:32:35 > 0:32:38but to knock the prey into the water,
0:32:38 > 0:32:40and they rarely fail.
0:33:25 > 0:33:28The seal is now where the killers wanted.
0:33:30 > 0:33:32But the hunt is far from over.
0:33:35 > 0:33:36They need to grab their prey by the tail,
0:33:36 > 0:33:39while avoiding its snapping jaws.
0:33:40 > 0:33:43Only then will they be able to pull it down and drown it.
0:33:47 > 0:33:52Side swipes create violent, underwater turbulence, a new tactic.
0:33:59 > 0:34:02Blowing bubbles gives cover for others to lunge at the seal's tail.
0:34:12 > 0:34:15Somehow, the seal manages to reach a tiny ice floe.
0:34:21 > 0:34:22The killers could easily grab it,
0:34:22 > 0:34:25but now, this seems to have become a game.
0:34:27 > 0:34:31The seal's life hangs on a roll of the ice.
0:34:44 > 0:34:47Yet again, the pod joins forces to dislodge the seal.
0:35:06 > 0:35:08The seal sees a chance to escape.
0:35:12 > 0:35:17Exhausted, it no longer has the energy to pull itself to safety.
0:35:17 > 0:35:19And the killers are moving in.
0:35:28 > 0:35:29Game over.
0:35:42 > 0:35:46As the Antarctic summer draws to an end, visitors that rely on the brief
0:35:46 > 0:35:52flush of food will soon be forced north by the return of the sea ice.
0:35:56 > 0:36:02For the largest land predator, the sea ice cannot return soon enough.
0:36:06 > 0:36:11The end of the Arctic summer and the sun hasn't set for three months.
0:36:14 > 0:36:17It's hard to imagine the bitter cold will soon return.
0:36:28 > 0:36:32Some will welcome the chills of autumn but, for most,
0:36:32 > 0:36:36time is running out before they will have to retreat to the south.
0:36:43 > 0:36:46The shorter days and colder nights trigger
0:36:46 > 0:36:50a dramatic change in the willows and blueberry bushes.
0:36:50 > 0:36:53They stop producing the green pigment that harnesses
0:36:53 > 0:36:58the sun's energy, and red and yellow pigments build up in their leaves.
0:37:07 > 0:37:10The tundra blazes with colour
0:37:10 > 0:37:12and the whole landscape is transformed.
0:37:33 > 0:37:35The sound of autumn.
0:37:35 > 0:37:37The musk ox rut has begun.
0:37:45 > 0:37:48This clash of polar titans could easily become a fight
0:37:48 > 0:37:52to the death, as males battle over females.
0:37:55 > 0:37:58A heavy skull and helmet of horn, four inches thick,
0:37:58 > 0:38:02provide some protection for the musk ox's brain.
0:38:12 > 0:38:15The impact is like a car crash at 30 miles an hour.
0:38:34 > 0:38:36Over the next half hour,
0:38:36 > 0:38:40the advantage ebbs and flows, as each tries to finish the duel.
0:38:40 > 0:38:42If one can turn the other
0:38:42 > 0:38:44and gore its flank, the fight will be over.
0:38:57 > 0:39:00The stakes rise as the risk of injury, or even death, increases.
0:39:14 > 0:39:16This male is outmanoeuvred.
0:39:18 > 0:39:20And the champion returns to claim his females.
0:39:25 > 0:39:29By now, most animals have migrated south.
0:39:29 > 0:39:31But the musk ox, with their thick coats,
0:39:31 > 0:39:34will stay and face the approaching winter.
0:39:43 > 0:39:47The moisture in the air freezes and hoarfrost decorates the leaves.
0:39:51 > 0:39:56Ice crystals grow like shards of glass on every twig.
0:40:11 > 0:40:16As the sun's influence continues o dwindle here in the north,
0:40:16 > 0:40:19the freeze pushes into the tree line,
0:40:19 > 0:40:21spreading over one fifth of the planet.
0:40:31 > 0:40:34The northern forests lock down for winter.
0:40:45 > 0:40:49The last running water freezes and cathedrals of ice are formed.
0:40:56 > 0:41:00The great waterfalls of the north shut down.
0:41:07 > 0:41:11The stage is now set for a magical event.
0:41:12 > 0:41:15The formation of snowflakes.
0:41:17 > 0:41:20Cold air meets warm air from the south
0:41:20 > 0:41:23and the moisture it carries crystallises.
0:41:31 > 0:41:33All have a six-fold symmetry,
0:41:33 > 0:41:36but no two have ever been found with exactly the same shape.
0:41:44 > 0:41:49Each snowflake is water waiting to be released in spring.
0:41:49 > 0:41:54For this reason, snow is the life blood of these silent forests
0:41:54 > 0:41:58and all that live here depend on it in one way or another.
0:42:05 > 0:42:08Some, like the Great Grey Owl,
0:42:08 > 0:42:13appear in spring for the boom times, then vanish like phantoms.
0:42:57 > 0:43:02Stooped, shrouded figures bear the weight of winter's heavy snows.
0:43:40 > 0:43:41The heavy snows make it
0:43:41 > 0:43:42hard to get around,
0:43:42 > 0:43:44even for the largest and
0:43:44 > 0:43:46most powerful wolves in the world.
0:43:48 > 0:43:51Today, they are setting out to hunt.
0:43:51 > 0:43:53The pack is 25-strong,
0:43:53 > 0:43:57a sign that the prey they are seeking is formidable.
0:43:57 > 0:43:59The largest land animal in North America.
0:44:00 > 0:44:02Bison.
0:44:13 > 0:44:16The bison form a defensive circle around their young,
0:44:16 > 0:44:17horns pointing outwards.
0:44:25 > 0:44:28The wolves need a bison to break rank.
0:44:34 > 0:44:38But the tables are turning and now the wolves have to retreat.
0:44:41 > 0:44:44The pack focus their attention on the rear of the herd
0:44:44 > 0:44:46and the bison begin to panic.
0:45:21 > 0:45:23A young bison falls behind.
0:45:30 > 0:45:32Even this yearling dwarfs the wolves.
0:45:40 > 0:45:42Running head down,
0:45:42 > 0:45:44the herd's only thought is escape.
0:45:49 > 0:45:51A stroke of luck for the wolves.
0:46:05 > 0:46:08The kill will feed the pack for several days.
0:46:08 > 0:46:12But then, they will have to resume the chase.
0:46:12 > 0:46:14At the frozen ends of our planet,
0:46:14 > 0:46:17the struggle for survival never eases.
0:46:28 > 0:46:30For a few, the snow is an ally.
0:46:38 > 0:46:43Voles stay active throughout the winter, moving between pockets
0:46:43 > 0:46:48of perfectly refrigerated food in a network of tunnels.
0:46:48 > 0:46:51These tunnels are a sanctuary from predators.
0:46:54 > 0:46:56Except one.
0:47:00 > 0:47:02The least weasel is a tiny hunter
0:47:02 > 0:47:04and the vole's nemesis.
0:47:11 > 0:47:13Its body is exactly
0:47:13 > 0:47:15the same width as a vole's,
0:47:15 > 0:47:18so there's nowhere a vole can go that a weasel can't follow.
0:47:46 > 0:47:50The weasel's long slender shape is perfect for hunting in tunnels,
0:47:50 > 0:47:53but the worst possible shape for staying warm.
0:47:53 > 0:47:56So, they need a special way of doing that.
0:47:58 > 0:48:01She plucks the fur from its body, tuft by tuft.
0:48:11 > 0:48:15And now, she puts it all together to make a cosy blanket,
0:48:15 > 0:48:17under which to sleep in her den.
0:48:34 > 0:48:39100 miles above the Earth, the aurora lights up the sky.
0:48:51 > 0:48:55After travelling millions of miles across space, solar winds, attracted
0:48:55 > 0:49:00by the magnetic pull of the Poles, collide with the Earth's atmosphere.
0:49:02 > 0:49:06Trillions of charged particles dance across the sky.
0:49:13 > 0:49:18Above the Arctic, the aurora borealis, the northern lights.
0:49:18 > 0:49:22In the south, it's the aurora australis,
0:49:22 > 0:49:26the southern lights, that bring light to Antarctica's long winter.
0:49:35 > 0:49:40These spectacular light shows are only a tease.
0:49:40 > 0:49:44Solar energy maybe, but no warmth that will help the Emperor Penguins.
0:50:05 > 0:50:09The male penguins have not eaten for months and have only each other
0:50:09 > 0:50:12for protection from hurricane-force winds.
0:50:17 > 0:50:21They have been deserted by the females, left to incubate
0:50:21 > 0:50:24their eggs throughout the worst winter on Earth.
0:50:36 > 0:50:39The Emperors are not entirely alone.
0:50:43 > 0:50:44The Weddell seal.
0:50:44 > 0:50:49The only mammal to remain here throughout the winter.
0:50:55 > 0:51:00Beneath the ice, they are beyond the reach of the bitter winter winds.
0:51:08 > 0:51:12The sea is minus two degrees centigrade.
0:51:12 > 0:51:15A warm bath compared to the conditions overhead.
0:51:23 > 0:51:25The roof of ice insulates this world
0:51:25 > 0:51:27from the wild fluctuations above.
0:51:29 > 0:51:34The temperature down here has barely changed for 25 million years.
0:51:46 > 0:51:49There could hardly be a greater contrast
0:51:49 > 0:51:52to the bleak, windswept world just above.
0:52:05 > 0:52:08But there is a constant danger here.
0:52:12 > 0:52:16Swirling patterns in the water reveal its presence.
0:52:18 > 0:52:23They're made by brine, super-concentrated salt water.
0:52:23 > 0:52:24It's a warning.
0:52:29 > 0:52:32New sea ice forming above leaves behind brine
0:52:32 > 0:52:36that is so extremely salty, it sinks rapidly.
0:52:42 > 0:52:46As it descends, the sea water around it freezes instantly
0:52:46 > 0:52:48and forms a sheath of ice.
0:52:48 > 0:52:52A brinicle that grows downwards towards the sea floor.
0:52:59 > 0:53:02Winter is reaching down from the cold world above.
0:53:23 > 0:53:26As it touches the sea floor, it kills whatever living thing
0:53:26 > 0:53:29it contacts by encasing it in a tomb of ice.
0:53:43 > 0:53:45As calm returns beneath the ice,
0:53:45 > 0:53:49a dramatic change is coming to the world above.
0:54:16 > 0:54:18The sun returns to Antarctica.
0:54:21 > 0:54:24The longest night on Earth has ended
0:54:24 > 0:54:26and winter begins to give way to spring.
0:54:47 > 0:54:50Female Emperor Penguins.
0:54:50 > 0:54:55After four months feeding at sea, they're returning sleek and fat.
0:55:25 > 0:55:28Penguins, it seems, can fly after all.
0:55:32 > 0:55:35But a winter at sea has left them a little out of practice.
0:56:28 > 0:56:34There is no time to waste. Faraway, the males are waiting.
0:56:47 > 0:56:50Reunited after three months apart.
0:56:56 > 0:56:59The reward for the female's return,
0:56:59 > 0:57:02a first glimpse of her chick.
0:57:08 > 0:57:12A task that began in autumn has been completed.
0:57:15 > 0:57:17Despite the huge odds against it,
0:57:17 > 0:57:21the precious chick has survived the winter and is now with its mother.
0:57:22 > 0:57:24And she has food.
0:57:30 > 0:57:32The chick's first fresh meal.
0:57:34 > 0:57:36The female's mission is complete.
0:57:44 > 0:57:47For those Emperor Penguins that survive,
0:57:47 > 0:57:48the worst is over for this year.
0:57:56 > 0:57:59There will soon be abundant food for everyone.
0:58:05 > 0:58:10The Emperors have taken on the polar winter and won.
0:58:10 > 0:58:11The gamble has paid off.
0:58:13 > 0:58:15All other animals escaped.
0:58:15 > 0:58:17Only they remained with their eggs,
0:58:17 > 0:58:23and it's they who will benefit most from the rich southern spring.
0:58:23 > 0:58:25Their epic journey is complete.
0:58:35 > 0:58:39Subtitles by Red Bee Media Ltd