In 1946, the United States began testing atomic bombs in an area known as the Pacific Proving Grounds. The Pacific Proving Grounds included a number of sites located at the Marshall Islands, a remote group of islands in the Central Pacific Ocean. From 1946 until 1963, the United States detonated 105 nuclear weapons at the site. While this only composed 14% of all nuclear testing conducted by the U.S., the testing at this site included 80% of the total energy yield from all nuclear weapon tests. In other words, the atomic weapons tested near the Marshall Islands were vastly more powerful and dangerous than weapons tested in mainland U.S.
Of the 105 devices tested, the most powerful was code named Castle Bravo. Part of Operation Castle, this was the first dry fuel hydrogen bomb detonated by the United States. It was detonated at Bikini Atoll, Marshall Islands. Its yield was equivalent to 15 megatons of TNT. To put that into perspective, this is 1,000 times more powerful than the atomic bombs dropped on Hiroshima and Nagasaki during World War II. Due to an error in calculation, the strength of the bomb was three times what scientists predicted. As a result, nuclear fallout in the surrounding Marshall Islands was much worse than anticipated.
This miscalculation in yield along with a major wind shift caused the radiation cloud to spread over an area 100 miles long Within this area were 15 islands home to nearly 20,000 people. Evacuations of the islands occurred 48 hours after detonation, however residents had already been exposed to dangerous levels of radiation. In the years that followed, hundreds of cases of radiogenic cancers and serious birth defects were reported by those exposed to the fallout. To this day, residents refuse to return to their former homes due to fear that the area is still unsafe.