The Marshall Islands atoll where the United States conducted nuclear tests is Bikini Atoll.
Bikini Atoll lies in the Ralik Chain of the Marshall Islands and consists of a coral reef surrounding a large lagoon. After World War II, the United States selected it as a test site because it was remote from major shipping and air routes. The atoll's residents were relocated in 1946 before testing began.
The first tests were Operation Crossroads detonations in July 1946, carried out to study the effects of nuclear weapons on warships. In total, the United States detonated 23 nuclear devices at Bikini between 1946 and 1958. The most powerful was Castle Bravo, a 1954 thermonuclear test that produced a 15-megaton yield, far above its predicted strength.
Radioactive fallout contaminated the atoll and affected nearby communities, making permanent resettlement unsafe. Bikini Atoll became a UNESCO World Heritage Site in 2010 because it preserves evidence of the nuclear age and its consequences. Enewetak Atoll was another Marshall Islands test site, but it is not the answer to this question.