Caribou

A hundred thousand caribou must face hungry bears and wolves, deadly frozen rivers and rugged mountains as they undertake the world's longest land migration - 3,000 miles through the frozen Arctic wilderness of Canada and Alaska.

The grizzles have just woken up from 6 months of hibernation, and they're starving. They badly need protein, and when the caribou pass through, they try to make the most of the opportunity to supplement their diet of roots and berries. The result is an area with one of the highest concentrations of grizzly bears on the planet.

With just 13 days left until much of the herd is due to give birth, this herd still has 180 miles to go. But the herd finally arrives where the team predicted. They descend from the mountains, spilling into the valley by the thousands.

In open landscape, a bear has a hard time catching caribou - but not humans. The camera crew must carefully remove themselves from the scene as the bear meanders, then charges, toward them.