S1, E2: As the spring melts the winter snow, the full extent of Yellowstone is gradually revealed. Now, from the surrounding lowlands herds of elk, pronghorn and bison return from their winter feeding grounds to take advantage of America's richest natural grasslands. Grizzly bears emerge from hibernation to teach their young to hunt fish and how to survive in the Alpine peaks. But just as it seems the easy living of summer has arrived, a new danger emerges.