Midway down the wild and roadless Alaska Peninsula lies one of the nation's most fascinating recent volcanic features. Aniakchak is a 6-mile-wide, 2,000 foot deep caldera formed by the collapse of a 7,000 foot mountain. Lying inland in a region of frequent clouds and stormy weather, Aniakchak remained unknown to all but Native inhabitants until the 1920s.
Although a dozen calderas stand on the Alaska Peninsula, Aniakchak ranks among the largest. About 3,500 years ago, a dramatic explosion caused the loss of some 3,000 feet of the upper mountain. The remainder of the mountain collapsed, leaving a relatively flat-floored, ash-filled bowl. Since the caldera first formed, many lesser eruptions -- the most recent in 1931 -- have created the small cinder cones, lava flow, and explosion pits dotting its floor today.
The caldera probably once contained a deep snow-fed lake, much like Crater Lake in southern Oregon. Eventually, the stream outflow created the great breach in the rim known as The Gates. Surprise Lake still remains, and from it The Gates now allow the Aniakchak River to begin its tumultuous 27-mile course southeastward to the Pacific Ocean.