The Aniakchak Caldera, is the result of a series of eruptions, the latest in 1931. Nearly six miles in diameter and covering some ten square miles, it is one of the finest examples of dry caldera in the world. Located in the volcanically active Aleutian Mountains, the crater contains many outstanding examples of volcanic features, including lava flows, cinder cones, and explosion pits. Surprise Lake, located within the caldera, is the source of the Aniakchak River, which cascades through a 1,500-foot gash in the caldera wall. The site also contains the Aniakchak Wild River.