Recent multi-disciplinary studies by the U.S. Geological Survey conducted on and near Amchitka Island in the Aleutians have contributed much to a better understanding of the geologic, hydrologic, and tectonic environment of the island as an underground nuclear test site. The work included geologic mapping, isotopic dating, deep drilling, geophysical surveys, hydrologic investigations, and post-test effects examinations. Three distinct volcanic episodes have been distinguished and dated. A general volcano-tectonic history of the arc is proposed. The islands in the vicinity of Amchitka have apparently been stable tectonically in the late Pleistocene and Holocene. Most of the gravity and magnetic data can be interpreted in terms of known structure and rock distribution. Marine geophysical data tend to confirm structural stability of the insular area and indicate a scarcity of sedimentary material on the insular slopes and shelf. Geologic effects of underground tests have been minor.