On the morning of March 1, 1954, a hydrogen bomb, code named Bravo, was detonated in the northwestern corner of Bikini Atoll. There was a flash of blinding light and a fireball moved to the sky at a rate of 300 miles an hour. In minutes a nuclear debris filled cloud went up more than 20 miles.

Shortly before the Bravo test the US Department of Interior changed the official "danger zone" and placed Ronelap, Rongerik, and Alinginae immediately outside the area that would necessitate evacuation, despite the fact that the planned detonation was designed to produce heavy fallout and in fact involve the largest nuclear device ever tested by the US government. The weather had been monitored for several days prior to the test and indicated the high likelihood that wind would blow radioactive fallout to the inhabited atolls of Rongelap, Rongerik and Ailinginae.

The previous tests in the Marshall Islands and the United States led to development of safety precautions for the minimizing of exposure to fallout. The military personal were given radiation information and instillations built near the test site were made to mineralize exposure. The people of Rongelap did not have access to protective shelters or safety information and had not been notified there was going to be a test that day.

When high levels of fallout were detected the military personal that were monitoring weather were instructed to cease operations immediately and enter metal-lined concert bunkers. They were evacuated on March 2, 1954. The Marshallese were not evacuated until March 04, 1954.

The people residing on these atolls were exposed to an estimated 200 roentgens of whole body-radiation with substantially greater amounts taken up by the thyroid. Between the time of the Bravo test and their evacuation the Rongelapsese ate food and drank water that was contaminated from fallout. Tests performed after the evacuation show that the water was radioactive at two to twenty five times the AEC's operational tolerances. Hours after the Bravo test, the Rongelapsese began to show flulike symptoms including nausea, diarrhea, and vomiting. By evening, blisters began to form on areas of skin that had been touched by fallout ash and people began lose clumps of their hair.