The mountain spine of the Aleutian Range consists of uplifted mountains of moderate height, reaching . The mountain environment is predominantly alpine tundra. Superimposed on the mountain chain are a series of volcanoes, the largest of which is the remnant of Mount Aniakchak, now largely collapsed into its caldera, the floor of which lies about above sea level. The caldera was once partly filled by a crater lake that covered about 50% of the crater. The caldera wall was eventually breached, leading to a catastrophic flood as the lake drained. Its remnant is Surprise Lake, the source of the Aniakchak River.