One hundred thousand caribou face starving bears and wolves, deadly frozen rivers and rugged mountains, as they undertake the world's longest land migration - 5000 kilometres through the frozen Arctic wilderness of Canada and Alaska.

Liz Bonnin leads a team of scientists and film-makers and uses cutting-edge technology to follow the herd, and reveal unique insights into the individual lives of these extraordinary animals.

The caribou must reach their calving grounds before they give birth; failure would spell disaster.