Cannikin was an underground nuclear weapons test performed on November 6, 1971, on Amchitka island, Alaska, by the United States Atomic Energy Commission. The experiment, part of the Operation Grommet nuclear test series, tested the unique W71 warhead design for the LIM-49 Spartan anti-ballistic missile. With an explosive yield of almost 5 megatons of TNT (21 PJ), the test was the largest underground explosion ever detonated by the United States.