Region of volcanic cones and craters, south-central Idaho, U.S. Established in 1924, it covers an area of 53,545 acres (21,669 hectares) and has more than 35 craters, probably extinct only a few centuries.
Some are nearly a half-mile across and several hundred feet deep and reach a height of more than 6,000 ft (1,800 m). Tunnels formed by fissure eruptions feature stalactites and stalagmites in red and blue.