Today we"re in Kobuk Valley National Park in Alaska to witness one of the world’s great annual migrations. Every spring, a quarter million caribou come together to form the Western Arctic Herd and pass north through the 1.8-million-acre park to their summer calving grounds in the Brooks Range and its foothills. Then in the fall, the caribou make the reverse trip, heading south back through the park to where they spend the winter in the Nulato Hills and the Seward Peninsula. One of the best spots to see the herd on the move is where it crosses the Kobuk River at Onion Portage. This area is a major archaeological site too. For thousands of years, the crossing has drawn native groups that rely on caribou meat, a tradition that continues to this day.