The largest underground nuclear test conducted by the United States, Project Cannikin was one of three underground nuclear tests performed at different places on this 42-mile long island in the Aleutian Chain. This $200 million 1971 test was performed to test an Anti-Ballistic Missile warhead, for a Spartan ABM missile. It consisted of a 5 megaton-yield thermonuclear bomb, detonated in a 50-foot diameter chamber, at the bottom of a 5,875-foot shaft. The resulting explosion was said to have had the force of a 7.0 magnitude earthquake, as measured on the Richter scale. The island of Amchitka was a military outpost in WWII, and the air field and base camp from that facility were reused for the nuclear testing program.
Cannikin Nuclear Test Site, Alaska