"In the winter of 1807, a lone fur-trapper journeyed deep into the heart of the Rocky Mountains." "Somewhere near the headwaters of the Yellowstone River ... he found a lost world." "A wonderland, ruled by ice, fire and brimstone." "A world of extremes that challenges all that strive to live here." "A place that has become perhaps the most treasured wilderness on Earth." "Winter in Yellowstone." "Minus 40 degrees." "Fahrenheit or centigrade, it doesn't really matter, at minus 40 the two scales read the same." "For half the year, Yellowstone is frozen solid." "Yet in the middle of this ice world there is scalding heat." "This is no ordinary place and this is no ordinary winter." "The fate of everything here lies in the hands of forces ... of almost unimaginable power." "Yellowstone is deep in the heart of the Rocky Mountains of North America." "An isolated high plateau defended by rugged peaks." "And its location is what makes it so different." "Right beneath Yellowstone a unique quirk of geology means that molten rock from deep in the earth comes unusually close to the frozen surface." "No one knows why it happens right here, but its impact is what has made Yellowstone world famous." "Yellowstone is the most extensive geothermal area on Earth." "It has over 10,000 thermal wonders ... and more geysers than the rest of the world put together." "Old Faithful is Yellowstone's most well-known geyser." "It shoots 5,000 gallons of water ... 150 feet into the air almost every hour." "But the forces that fuel this spectacular display ... have an influence far greater than we can see on the surface." "Paradoxically, it's all this underground heat ... that helps make the Yellowstone winter ... one of the coldest and toughest in America." "It's November and winter is beginning to take hold." "As it gets colder, one animal here gets stronger." "Wolves." "The winter is their time." "Gradually, it weakens their prey." "This is the Druid wolf pack, one of the largest and most powerful in Yellowstone." "The pack have this bull elk surrounded." "But there's a problem." "The pack won't follow the bull into the river." "They won't risk freezing to death in the ice cold water." "What's more, now the elk's antlers are at just the right height to keep the wolves at bay." "It's stalemate." "But it's now the elk that has a problem of his own." "Although it's only knee deep, he can't stay in this freezing water forever." "A young female is not prepared to let him go." "But the elk is strong." "One-on-one he has the advantage." "Her only support is another youngster." "They are neither strong or experienced enough ... to bring this elk down." "But it's enough to make him turn and run ... back to the river where he knows they won't follow." "But the longer he stays in the freezing water, the weaker he will get." "Others before him have waited here too long, and wolves are patient." "Right now his strength is his only advantage." "He has to try again." "This time even the young wolves stay put." "Without the support of the pack they never really stood a chance." "And the pack have already decided that this early in the winter, a bull elk in his prime is just too strong." "But as the winter gets colder ... and the snow gets deeper the tables will turn." "By the end of November, the arc of the sun barely breaks above the trees." "As its angle decreases, so does its power." "And as the sun loses its hold over the land, other forces begin to take over." "Yellowstone has a dark secret ... that affects everything that lives here, especially in the winter." "It's only from high above ground ... that we start to get a glimpse of the true nature of this place." "Yellowstone is a giant bowl 50 miles wide ... right in the middle of the Rocky Mountains." "There's nowhere else like it." "And there's only one thing that could have created it." "Three miles beneath this frozen surface ... is a colossal chamber of molten rock." "Today it powers Yellowstone's geysers." "But every million years or so, the pressure in this magma gets critical and the chamber explodes." "The last eruption, 640,000 years ago, was more than 1,000 times larger than Mount St Helens." "It blasted away mountains and ejected hundreds of cubic miles of debris ... into the atmosphere, burying half the USA with ash." "The heart of Yellowstone is one of the world's biggest volcanoes." "One day it will erupt again." "It could be today or in another million years." "But even as the volcano is sleeping, breathing quietly through its geysers, it has a profound effect on Yellowstone's winter." "The volcano made Yellowstone's giant bowl but it didn't stop there." "Ever since, the huge pressure below the surface ... has been pushing it higher into the air, and as it gets higher it gets colder." "And now at its present altitude of 8,000 feet, this giant bowl simply accumulates freezing air ... from the surrounding mountains." "In the winter, the sleeping volcano becomes a giant deep freeze." "On the open plateau, right in the middle of this frozen volcano ... is an animal that has lived here since the last ice age." "Bison are exposed to the worst of the Yellowstone winter, but they are built for it." "Their thick coat is such good insulation ... that they only need a tiny amount of energy to keep warm." "So they slow their metabolism right down and concentrate on feeding." "With massive neck muscles they sweep their heads down through the snow ... to get to the grass beneath." "But the grass has long ago put its summer goodness down into its roots ... and now has about the same nutritional value as cardboard." "They will need to do all they can to save energy ... if they are to ward off starvation until spring returns." "As the winter strengthens its grip, elk move into more sheltered valleys at the edge of Yellowstone." "They don't have the bison's ability to move deep snow." "But this brings them into the territory of the Druid pack." "As the grazers are beginning to weaken, life for the wolves is getting easier." "They are now successfully hunting about twice a week." "They even have the energy to play." "But their play has a purpose." "It fine-tunes their hunting skills ... and helps bond the all-important pack structure." "Though there are 16 of them, they can only hunt an animal as large as an elk if they hunt as one." "The strength of the pack is what will get them through the winter." "Bald eagles spot carcasses from miles away." "But there is strong competition for a kill like this." "A coyote." "He has been shadowing the wolves, and moves in now they have gone." "It's December, and even the great Yellowstone River ... is succumbing to the cold." "It's only where the water runs fast that it still runs free." "It looks uncomfortably cold, but then the water, at around freezing point, can be 50 degrees warmer than the air." "Under the ice there's a rich supply of stone fly larvae ... waiting to hatch in the spring." "Dippers make the most of these few small windows to a liquid world ... before they shut completely." "Where the water stands still it is now frozen solid." "Yellowstone Lake is 136 square miles, and now completely covered in three feet of ice." "A coyote travels across this frozen desert looking for something to eat." "It's a wonder that anything can survive here at all." "Hundreds of feet beneath him on the lake bed, geysers erupt just like they do on land ... and they melt holes in the ice, the only sign that there is a lake here at all." "As the year comes to an end, it seems hard to imagine this winter getting any tougher." "But there's another twist to the volcano's story ... that is about to make things even worse." "Over time the continent of North America has moved, inch by inch, over many millions of years." "But deep down below the Earth's moving crust, the source of magma that fuels Yellowstone's volcano ... has stayed put." "As the crust has moved over this volcanic hotspot, eruption after eruption has blasted a massive 500-mile-long scar ... right through the Rockies." "In the winter this giant scar, called the Snake River Plain, funnels moist air from the Pacific Ocean ... right through the wall of the Rocky Mountains ... and up into Yellowstone's deep freeze." "Here it finally freezes and falls as snow, huge quantities of it." "Whilst everywhere around gets 10 feet of snow a year, thanks to the legacy of its volcano, Yellowstone can get as much as 50." "Otters seem to thrive in the Yellowstone winter." "But now that the rivers are not only frozen but covered in deep snow, they are struggling to find open water to fish in." "They can't fish here, the fast flowing water is too dangerous." "Somehow they need to find a way past the falls." "With the falls safely behind them, the otters are forced to keep moving on." "Open water has become a rare thing in Yellowstone." "Out on the frozen grasslands, the bison are struggling, too." "This year is already the snowiest for the last decade, snowier than many of this herd have experienced in their lives." "Now, as the snow gets deeper than a critical four feet, the effort of swinging this massive head back and forth for so little reward ... is becoming too much." "Though the snow front is passing through, it's followed by the wind, which now starts to scour the heart of Yellowstone." "A bison's coat can keep it warm down to minus 30 degrees Fahrenheit." "The wind chill is now pushing these bison to that limit." "But these are the last wild bison herds in America ... which have survived here for tens of thousands of years." "They know what to do." "To move is risky, it will tap into their now dwindling energy reserves." "But this year, it's a gamble worth taking." "Their way out is a river whose water is not frozen." "A thermal river fed by warm water from Yellowstone's geysers, an ancient route which leads to a place where, if they're lucky, they will be able to survive." "It's January, and night is twice as long as day ... in Yellowstone's deep freeze." "The wind and the storms have gone, but now the clear skies suck any last trace of heat into space." "As morning comes, something extraordinary has happened." "All moisture in the air has turned to ice, diamond dust." "But this is a cruel beauty." "Minus 66 Fahrenheit was recorded here in Yellowstone." "Off the record, it gets colder still." "This bison is still out on the open plateau." "The deep snow with its windblown crust has made it almost impossible to feed." "The extreme cold will now tip the balance of survival further, most likely too far even for a bison." "A red fox can stay in the cold heart of Yellowstone all winter, so long as it can find food." "It's looking for mice ... that survive the winter insulated beneath the blanket of snow." "The fox is light enough to move about on the delicate crust ... but the mice are six feet beneath it." "It listens for the tiny sounds of its prey moving about below, but must take great care not to scare them away." "The otter family has arrived at Yellowstone Lake." "Here they can fish in the holes kept open by the underwater geysers." "But every time they catch something," "this coyote has been watching and waiting." "The otter dives under the ice to hide its fish from the coyote." "The coyote can't see the otter because of the thick cover of snow." "But he can hear him." "The otter emerges without the fish." "He's stashed it somewhere under the snow." "But where?" "A huge Yellowstone cutthroat trout." "With the help of the otters, a wily coyote can catch fish, too." "The thermal river has led the bison to one of the main geyser fields." "Here the heat from below comes close enough to the surface to melt the snow." "And a bison can graze as if it were spring." "The same volcanic forces, so massive that they created the weather ... that drove the bison here, now offer comfort." "The only problem is, the grass that the bison now relish ... has such a high concentration of silica that it wears down their teeth." "And it's laced with enough arsenic to slowly poison them." "For these bison, it's not an easy choice to come here." "But as long as they don't have to stay here too long ... it's a lot better than facing the Yellowstone winter head on." "Incredibly, there is life that thrives here." "In Yellowstone's thermal springs, the temperature is a constant near-boiling." "Yet here are huge colonies of heat-tolerant microbes." "As the boiling water flows out from the centre of springs, it cools, forming bands of different temperatures, each with a different collection of microbes ... with a totally different colour." "Grand Prismatic Spring is one of the wonders of the natural world." "It's thought that it was in conditions like this ... that life on Earth first started." "It's now February, and when almost everything else in Yellowstone is on its last legs, the Druid pack is reaching peak condition." "And it's now that the young females come into season." "Hanging back from the pack is a lone male wolf." "He has no territory of his own but follows the pack, scavenging from their successes." "But right now, food is not his priority." "The young females won't mate with the pack's alpha male ... as he is their father." "So the intruder could well be in with a chance, as long as the alpha doesn't see him." "Whilst the pack are distracted, one female sneaks away." "She won't give up the security of the pack for him, so they meet close by in secret." "But the pack are now coming their way." "The alpha male is on to him." "He won't tolerate any other male in his territory, let alone with one of his females." "When wolves mate, they become locked together for up to half an hour." "The intruder can't break free." "For now the alpha male has done enough." "He's seen the intruder off and he needs to return ... to reassert his position in the pack." "The intruder retreats to a precarious life in the shadows." "But whatever happens to him, so long as he was coupled for long enough, he will have young brought up ... in the security of one of the strongest packs in Yellowstone." "Since the beginning of winter," "Yellowstone's herds have been getting steadily weaker." "Now at the end of February, the tables have completely turned to favour the Druid pack." "As February turns to March, it seems like the winter will never end." "But now the clear, cold days have gone." "The snow still comes, but it's a wet snow ... that strips the warmth from you faster," "now, when you are right at the end of your strength." "But there is hope." "Now is the turning point of the winter." "At the spring equinox, there are 12 hours of night, and 12 hours of day." "From now on, light starts to win over dark." "March is also the turning point in the history of Yellowstone." "On 1st March, 1872," "American President Ulysses S. Grant ... recognised the extraordinary wonders of Yellowstone ... by making it the world's first national park." "The park's creation marked the beginning of a new era ... where the world's wild places would be valued ... simply for being wild." "Now, as the days lengthen, the winter starts to loosen its grip on Yellowstone." "But with the end of winter, also comes the end of the wolves' reign." "On a mountain peak right on the edge of Yellowstone, footprints in the snow are the sign that a challenger has appeared ... to reclaim this land." "A grizzly bear mother with her new cubs emerges from her den." "For six months, snow and ice have ravaged Yellowstone, but she has slept underground, waking only to give birth to her cubs." "And then from time to time, to feed them." "As winter gives way to spring, she leads them out into the wilderness for the first time." "In Yellowstone's great volcano, in spite of everything the winter has thrown at them, most have made it through." "The forces that have helped keep Yellowstone ... in the grip of such a deep winter have finally let go." "It is the sun that will now dominate once more." "Its power will now take over," "bringing new life to this place." "But also it will bring new challenges ... that all will have to face ... in the heat of Yellowstone's summer."