Amchitka
There were nuclear weapons tests at all of these: Christmas Island (by the British) 1957/58; Bikini Atoll (by the USA) 1946-58 but none in-ground; Mururoa Atoll (by the French) 1966 to 1996. The 1971 test, codenamed Cannikin, on Amchitka in Alaska was 250 times the yield of the bomb that was dropped on Nagasaki.