From 1946 to 1958, as the site of the Pacific Proving Grounds, the U.S. tested 67 nuclear weapons in the Marshall Islands, including the largest nuclear test the U.S. ever conducted, Castle Bravo. In 1956, the United States Atomic Energy Commission regarded the Marshall Islands as "by far the most contaminated place in the world".
Nuclear claims between the U.S. and the Marshall Islands were ongoing, and health effects from these nuclear tests lingered, even after skydark. Project 4.1 was a medical study conducted by the United States of those residents of the Bikini Atoll exposed to radioactive fallout. From 1956 to August 1998, at least $759 million was paid to the Marshallese Islanders in compensation for their exposure to U.S. nuclear testing.