Craters of the Moon National Monument, 53,440 acres (21,636 hectares), S central Idaho; est. 1924. This region, composed of several closely grouped volcanoes, is suggestive of a telescopic view of the moon. Volcanic activity dating back c.20,000 years has left behind cinder cones, tree molds, craters, and other interesting formations. At one time Native Americans used the lava caves. See National Parks and Monuments (table).