The first explorers to penetrate beyond the headwaters of the Missouri brought back tales of a strange land. They spoke of valleys that seemed to smoke and burn perpetually, and of a river that never froze, even in the hardest winter. That land is Yellowstone, America's oldest national park. Two-and-a-half million visitors flock there each summer, but in winter the park takes on another face that most people never see. Bison huddle in the warmth of the hot springs and the steam vents and geysers attain a new beauty in the frozen air.