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Amchitka, a volcanic, tectonically unstable island, is a part of the Rat Islands group of the Aleutian Islands located in southwest Alaska. It is about 68 kilometres (42 miles) long, and ranges from 3 to 6 km (2–3.75 mi) in width. Courtesy of its maritime climate, it is particularly susceptible to storms, and is mostly shrouded with overcast skies. The island was inhabited for more than 2,500 years by the Aleut people, but has had no permanent population since 1832. It was included in the Alaska Purchase of 1867, and since then has been part of the United States. During World War II, it was used as an airfield by US forces in the Battle of the Aleutian Islands.

Amchitka was selected by the United States Atomic Energy Commission to serve as the site for underground detonations of nuclear weapons. Three such tests were carried out: Long Shot, an 80 kiloton blast in 1965; Milrow, a 1 megaton blast in 1969; and Cannikin in 1971 – at "under 5 megatons", the largest underground test ever conducted by the United States. The tests were highly controversial, with environmental groups fearing that the Cannikin explosion, in particular, would cause severe earthquakes and tsunamis. Amchitka is no longer used for nuclear testing, although it is being constantly monitored for the leakage of radioactive materials.