Geothermal activity comes in many colors in Yellowstone National Park

This spitting and gurgling zone of geological wonder in northwest Wyoming is the world's oldest national park. Here, geothermal activity bursts forth in glorious colors as rivers and lakes and rainbow-hued hot springs and to a soundtrack of subterranean rumblings, belching mudpots and steam hissing from its famed geysers.

Add to this the park's extraordinary concentration of wildlife - which includes bison, moose, elk, mountain lions, wolves, grizzlies, and coyotes - and it's not hard to see why three-and-a-half million visitors annually make the pilgrimage. This, unfortunately, has become the park's weakness, but if you manage to visit in fall (late August and September) as the foliage turns to dazzling shades of red and gold then you'll certainly miss most of the crowds.