The first of March 1954 marks one of the most serious nuclear fallout incidents in history. On this day, the United States conducted its largest ever nuclear weapon test, code-named Castle Bravo, at the Bikini Atoll in the Marshall Islands. Bravo was part of Operation Castle, a nuclear test series designed to develop an aircraft-deliverable thermonuclear weapon. Due to a design error, the explosion reached a yield of 15 megatons, making it two and a half times larger than expected and more than 1,000 times as powerful as the Hiroshima bomb. The largest-ever nuclear explosion was the 1961 Tsar Bomba with 50 megatons.

Radioactive fallout from the test spread over more than 11,000 square kilometres. Traces of radioactive material were detected in Australia, India, Japan, the United States and Europe. The Bikini population had been relocated to other atolls prior to the start of the U.S. nuclear testing programme in the Pacific with the Able test in 1946. Due to the unfavourable weather conditions in which the Bravo test had been conducted, the fallout also affected the inhabited atolls of Rongelap, Utrik and others.