region of volcanic cones (cinder cone), craters, and lava flows near the foot of the Pioneer Mountains in south-central Idaho, U.S., 18 miles (29 km) southwest of Arco. The craters (more than 35), which have probably been extinct only a few millennia, were part of a tract set apart as a national monument in 1924; some are nearly a half mile across and several hundred feet deep. The monument's area was expanded considerably in 2000—from 83 to 1,117 square miles (215 to 2,893 square km)—which brought all of the region's lava flows under federal protection. In 2002, 476 square miles (1,233 square km) of that area were designated a national preserve.

The Craters of the Moon lava field is the largest of its kind in the coterminous United States; it is part of the extensive, volcanic Snake River Plain that stretches across much of southern Idaho. The monument's name was suggested by the terrain's superficial resemblance to the Moon's surface. Some two dozen volcanic cones, scattered along the rift that crosses the monument from northwest to southeast, reach elevations more than 6,000 feet (1,800 metres) above sea level. Although the black lava and cinders support only the thinnest vegetation and quickly absorb the scanty precipitation, water can be found in the lava tubes, or tunnels, formed by fissure eruptions through a partly formed crust. Sagebrush and mock orange are typical shrubs, and wildflowers are abundant in summer. Lava stalactites and stalagmites in red and blue are striking features of the tunnels.