"The polar regions are the coldest, most extreme places on our planet." "Winds reach 200 miles an hour and temperatures drop as low as minus 70 degrees centigrade." "Only the toughest can survive here." "Over the course of a year, polar animals must overcome not just the cold, but the most extreme seasonal changes on Earth." "In the High Arctic, winter is one long night that lasts for months." "On lee-side slopes, beneath the snow, new lives are beginning." "The cubs are born blind and tiny." "An early birth is easier on the mother, who is barely awake." "Despite her sleepiness, her instinct to nurse is overwhelming." "CUBS SQUEAK" "The cubs' clucking calls stimulate her to produce milk." "And what milk!" "It's nine times richer than our own, and enables her to double their weight every few weeks." "The sun is returning after an absence of nearly four months." "And its warmth starts to transform this magical ice world." "Spring has arrived." "The greatest seasonal change on our planet is now under way." "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." "By fair means or foul, the males must finish their nests quickly if they're to raise a family over the short Antarctic summer." "In the north of our planet, the weak sunshine is a welcome relief after the months of darkness." "Finally, it's time for polar bear families to emerge from their dens." "This is not the easiest place to take your first steps." "And the little ones will need plenty of encouragement from their mother." "If she can raise all three to independence, it will be a rare achievement." "During the months underground, all they've known is their mother." "Now, the big, wide world holds all sorts of challenges for these brave, young explorers." "The mother leads her new family out over the frozen sea to hunt for seals." "But the spring melt is already under way." "All across the Arctic, the sun's warming effect is increasing." "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." "CREAKING" "The vast watershed lies motionless." "But as spring advances, it begins 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." "These vast floods accelerate the breakup." "Soon, an area of sea ice the size of Australia will vanish from the Arctic Ocean." "The spring melt breaks the ice for visitors." "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." "The melting sea ice reveals open ocean." "And the changes on land are no less dramatic." "Beneath the snow, life is already stirring." "The Arctic tundra is unveiled." "Water that has been locked in ice all winter once again flows freely." "A landscape that was only white now bursts with colour." "Migrants begin arriving from the south and, suddenly, the tundra is alive with new families." "These Arctic wolf cubs are just over a month old and always looking out for their next meal." "Their parents have been hunting." "Their offerings are devoured instantly." "Arctic hare is a mainstay of the tundra diet and one the cubs seem 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." "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." "But finding a decent meal here is never easy." "The parents travel up to 80 miles in a day in search of more substantial prey for their growing family." "Musk oxen are immensely powerful." "And their sharp horns can kill." "But a calf is more vulnerable." "The two wolves work together to split the herd and isolate their victim." "It seems that the wolf cubs will, at last, eat well." "But the herd regroups." "The cattle ride to the rescue." "The wolves cannot penetrate the wall of horns." "The herd are protecting their calf." "For the musk oxen, it's all for one and one for all." "It's a struggle for all polar animals to feed themselves and their families in the brief Arctic summer." "But there is one tiny creature of the tundra that has found an astounding solution to the shortness of the season." "The woolly bear caterpillar." "It's always the first insect to appear after the snow's retreat." "The caterpillar then eats as fast as it can and, indeed it must, for this far north, summer only lasts a few weeks." "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." "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 summer 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." "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 struggle to survive in this most seasonal of places." "In a matter of weeks, the north will be frozen over once again." "At the southern end of our planet," "Antarctica is still surrounded by sea ice." "The Antarctic spring arrives first at the outer islands." "Although the beaches of South Georgia are now ice-free, the King Penguins face a new challenge." "Their peaceful waterfront has turned into an obstacle course of blubber." "The elephant seals have arrived." "The beachmaster's authority is being challenged." "This rival means business." "The beachmaster owns the females here and must to fight keep them." "The beachmaster himself weighs four tonnes, but this rival is his equal." "The beachmaster 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." "In summer, the Southern Ocean bursts with life." "No bird is more at home in water, and they are masterful surfers." "Penguins are found only in the Southern Hemisphere." "They can't fly, but they don't need to." "There are no polar bears here." "These are Gentoo Penguins." "Each spring, they come ashore to lay their eggs and rear their young." "Their hungry chicks demand so much seafood that both parents have to go fishing." "And fishing can be dangerous." "A southern sea lion." "It uses the speed of a breaking wave to catch up with the Gentoos." "Sea lions normally eat fish, so he's used to catching streamline swimmers." "But the Gentoos seem more than his match out at sea." "He must change tactics." "Perhaps it would be easier in the shallows." "But, no, it seems penguins are uncatchable in water." "How about on land?" "The penguin's wings, so powerful for swimming, are of no help when it comes to running." "Now, surely, the sea lion has a chance?" "But on the beach, both are like fish out of water." "Rarely do hunter and hunted play their roles with so little skill." "The outcome is anyone's guess." "Every summer, over 40 million penguins take to the Southern Ocean to feed." "They're joined by thousands of whales." "Minkes are the most numerous." "They all come here to harvest the richest ocean on Earth." "Southern humpbacks have travelled 4,000 miles from the equator to get here in time for the Antarctic summer." "Each year, the sea ice that surrounds Antarctica melts, effectively halving the size of the continent and exposing rich waters around its coastline." "It's the most spectacular seasonal change occurring anywhere on our planet." "The remnants of the sea ice are occupied by sunbathing seals that have been here all winter." "But new arrivals are following the retreating ice edge." "And they have come here to hunt." "Killer whales." "The ocean's top predator." "Killers are like wolves, for they will hunt animals far larger than themselves." "But even smaller prey are a problem if you can't reach them." "The solution is teamwork." "Swimming in perfect formation, they flick their tails in unison and create a wave that cracks the ice." "They regroup and assess the damage." "A more powerful wave is needed." "The ice floe is breaking up." "Now, they are close enough to get a good look at their target." "The seal is a crabeater, sharp-toothed and feisty." "Not their favourite." "The wolves of the sea move on in search of easier quarry." "A Weddell seal, that's better." "These are more docile and easier to tackle." "The pod stays close together and travels silently." "This time, they unleash a far more powerful wave, and with astonishing accuracy." "These big waves are not intended to break the ice, but to knock the prey into the water, and they rarely fail." "The seal is now where the killers wanted." "But the hunt is far from over." "They need to grab their prey by the tail, while avoiding its snapping jaws." "Only then will they be able to pull it down and drown it." "Side swipes create violent, underwater turbulence, a new tactic." "Blowing bubbles gives cover for others to lunge at the seal's tail." "Somehow, the seal manages to reach a tiny ice floe." "The killers could easily grab it, but now, this seems to have become a game." "The seal's life hangs on a roll of the ice." "Yet again, the pod joins forces to dislodge the seal." "The seal sees a chance to escape." "Exhausted, it no longer has the energy to pull itself to safety." "And the killers are moving in." "Game over." "As the Antarctic summer draws to an end, visitors that rely on the brief flush of food will soon be forced north by the return of the sea ice." "For the largest land predator, the sea ice cannot return soon enough." "The end of the Arctic summer and the sun hasn't set for three months." "It's hard to imagine the bitter cold will soon return." "Some will welcome the chills of autumn but, for most, time is running out before they will have to retreat to the south." "The shorter days and colder nights trigger a dramatic change in the willows and blueberry bushes." "They stop producing the green pigment that harnesses the sun's energy, and red and yellow pigments build up in their leaves." "The tundra blazes with colour and the whole landscape is transformed." "The sound of autumn." "The musk ox rut has begun." "This clash of polar titans could easily become a fight to the death, as males battle over females." "A heavy skull and helmet of horn, four inches thick, provide some protection for the musk ox's brain." "The impact is like a car crash at 30 miles an hour." "Over the next half hour, the advantage ebbs and flows, as each tries to finish the duel." "If one can turn the other and gore its flank, the fight will be over." "The stakes rise as the risk of injury, or even death, increases." "This male is outmanoeuvred." "And the champion returns to claim his females." "By now, most animals have migrated south." "But the musk ox, with their thick coats, will stay and face the approaching winter." "The moisture in the air freezes and hoarfrost decorates the leaves." "Ice crystals grow like shards of glass on every twig." "As the sun's influence continues o dwindle here in the north, the freeze pushes into the tree line, spreading over one fifth of the planet." "The northern forests lock down for winter." "The last running water freezes and cathedrals of ice are formed." "The great waterfalls of the north shut down." "The stage is now set for a magical event." "The formation of snowflakes." "Cold air meets warm air from the south and the moisture it carries crystallises." "All have a six-fold symmetry, but no two have ever been found with exactly the same shape." "Each snowflake is water waiting to be released in spring." "For this reason, snow is the life blood of these silent forests and all that live here depend on it in one way or another." "Some, like the Great Grey Owl, appear in spring for the boom times, then vanish like phantoms." "Stooped, shrouded figures bear the weight of winter's heavy snows." "The heavy snows make it hard to get around, even for the largest and most powerful wolves in the world." "Today, they are setting out to hunt." "The pack is 25-strong, a sign that the prey they are seeking is formidable." "The largest land animal in North America." "Bison." "The bison form a defensive circle around their young, horns pointing outwards." "The wolves need a bison to break rank." "But the tables are turning and now the wolves have to retreat." "The pack focus their attention on the rear of the herd and the bison begin to panic." "A young bison falls behind." "Even this yearling dwarfs the wolves." "Running head down, the herd's only thought is escape." "A stroke of luck for the wolves." "The kill will feed the pack for several days." "But then, they will have to resume the chase." "At the frozen ends of our planet, the struggle for survival never eases." "For a few, the snow is an ally." "Voles stay active throughout the winter, moving between pockets of perfectly refrigerated food in a network of tunnels." "These tunnels are a sanctuary from predators." "Except one." "The least weasel is a tiny hunter and the vole's nemesis." "Its body is exactly the same width as a vole's, so there's nowhere a vole can go that a weasel can't follow." "The weasel's long slender shape is perfect for hunting in tunnels, but the worst possible shape for staying warm." "So, they need a special way of doing that." "She plucks the fur from its body, tuft by tuft." "And now, she puts it all together to make a cosy blanket, under which to sleep in her den." "100 miles above the Earth, the aurora lights up the sky." "After travelling millions of miles across space, solar winds, attracted by the magnetic pull of the Poles, collide with the Earth's atmosphere." "Trillions of charged particles dance across the sky." "Above the Arctic, the aurora borealis, the northern lights." "In the south, it's the aurora australis, the southern lights, that bring light to Antarctica's long winter." "These spectacular light shows are only a tease." "Solar energy maybe, but no warmth that will help the Emperor Penguins." "The male penguins have not eaten for months and have only each other for protection from hurricane-force winds." "They have been deserted by the females, left to incubate their eggs throughout the worst winter on Earth." "The Emperors are not entirely alone." "The Weddell seal." "The only mammal to remain here throughout the winter." "Beneath the ice, they are beyond the reach of the bitter winter winds." "The sea is minus two degrees centigrade." "A warm bath compared to the conditions overhead." "The roof of ice insulates this world from the wild fluctuations above." "The temperature down here has barely changed for 25 million years." "There could hardly be a greater contrast to the bleak, windswept world just above." "But there is a constant danger here." "Swirling patterns in the water reveal its presence." "They're made by brine, super-concentrated salt water." "It's a warning." "New sea ice forming above leaves behind brine that is so extremely salty, it sinks rapidly." "As it descends, the sea water around it freezes instantly and forms a sheath of ice." "A brinicle that grows downwards towards the sea floor." "Winter is reaching down from the cold world above." "As it touches the sea floor, it kills whatever living thing it contacts by encasing it in a tomb of ice." "As calm returns beneath the ice, a dramatic change is coming to the world above." "The sun returns to Antarctica." "The longest night on Earth has ended and winter begins to give way to spring." "Female Emperor Penguins." "After four months feeding at sea, they're returning sleek and fat." "Penguins, it seems, can fly after all." "But a winter at sea has left them a little out of practice." "There is no time to waste." "Faraway, the males are waiting." "Reunited after three months apart." "The reward for the female's return, a first glimpse of her chick." "A task that began in autumn has been completed." "Despite the huge odds against it, the precious chick has survived the winter and is now with its mother." "And she has food." "The chick's first fresh meal." "The female's mission is complete." "For those Emperor Penguins that survive, the worst is over for this year." "There will soon be abundant food for everyone." "The Emperors have taken on the polar winter and won." "The gamble has paid off." "All other animals escaped." "Only they remained with their eggs, and it's they who will benefit most from the rich southern spring." "Their epic journey is complete."