Documentary following 100,000 caribou as they face bears, wolves, frozen rivers and rugged mountains to undertake the world's longest land migration. Liz Bonnin and a team of scientists track the herd as they travel 5,000km through the Arctic wilderness of Canada and Alaska, revealing insights into the lives of individual animals as they attempt the epic journey. Cameras capture the caribou as they fight to reach their calving grounds before they give birth - failure would spell disaster