A nuclear test unleashed radioactive fallout across Pacific islands, leaving hidden damage that scientists are still tracing more than 70 years later. Radioactive powder began falling on Rongelap Atoll several hours after the United States detonated the Castle Bravo nuclear test on March 1, 1954. Credit: Marshall Islands ProgramWeapons designers then turned to lithium-based solid fuel in an effort to build a more transportable thermonuclear device. Castle Bravo became the largest of the 67 U.S. nuclear tests conducted in the Marshall Islands between 1946 and 1958. The vessel’s exposure brought the consequences of Pacific nuclear testing into public view in Japan and intensified concern about contaminated fish and radioactive fallout.