Yellowstone: geysers, bison and springs of every colour
The world's first national park, sitting on a volcano: geysers, multicoloured hot springs, a yellow canyon and bison in the middle of the road.
Craters of the Moon: an ocean of black lava in Idaho
Frozen lava flows as far as the eye can see, black cinder cones and tunnels to explore by torchlight. The landscape where the Apollo astronauts came to train.