Winter

In winter, Yellowstone is frozen solid - locked in snow as deep as a house for over six months. Whether you hunt for meat, live off stored body fat or whether you simply hibernate, you need to take everyadvantage, however slight, to save precious energy - then you might just make it through the winter to enjoy the green grass and balmy days of spring.

As we follow the grip of winter over the course of six freezing months, we chart the fortunes of Yellowstone's wildlife in a finely balanced fight to survive. Bison use their massively powerful heads to dig through some of the deepest snow in America to reach the grass beneath. A red fox listens out for mice scurrying six feet beneath the snow before diving headfirst into the drift to snap up its prey, while otters slide through Yellowstone's winter wonderland to find any remaining open water where they can fish. All the while, as the herds of elk and bison are gradually weakened by the cold, one animal gets stronger - the wolf.

But all is not as it first seems - there are larger powers at work. Whether a wolf, a bison or an elk makes it through is intimately linked to Yellowstone's greatest secret. Sleeping beneath the ice and snow-covered surface is one of the world's largest volcanoes. In an extraordinary twist of nature, everything from the freezing winter cold to the creation of a snowstorm is determined by the power of Yellowstone's volcanic heart.