At 6:45 a.m. on March 1, 1954, the U.S. detonated a thermonuclear weapon at Bikini Atoll in the Marshall Islands, a chain of 29 coral atolls and five islands in the central Pacific Ocean, about halfway between Hawaii and Australia. The test was code-named “Castle Bravo.”

This was during the early Cold War, when the U.S. and the Soviet Union were in a race with each other to build bigger, better, more destructive nuclear weapons. The U.S. had already tested its first full-scale hydrogen bomb in 1952 during Ivy Mike, a thermonuclear blast at Enewetak Atoll that destroyed the small island of Elugelab and left behind a mile-wide crater.

Now, the U.S. wanted a bomb small enough to be carried by an aircraft and actually used in war. This device was nicknamed the Shrimp, ironic for a bomb that exploded with 15 megatons of force—nearly three times more than expected, and about 1,000 times the force of the bomb dropped on Hiroshima.

The Shrimp had been placed in a small test structure on an artificial island built on the reef off Namu Island, part of Bikini Atoll. When it went off that morning, the blast tore through coral reef and seawater, vaporized millions of tons of sand, coral, and water, carved a crater into the seafloor of Bikini’s lagoon, and sent radioactive debris high into the sky.

Rongelap Atoll, a chain of coral islands around a lagoon, was about 110 miles from the test site. Approximately 80 Marshallese people on Rongelap and nearby Ailinginae would become the most heavily exposed civilian group from the test. No one had told them to evacuate beforehand because U.S. planners thought the fallout would miss the atolls where people lived or arrive much later, after most of the radioactivity had faded.

But that was a deadly assumption, as Sarah Scoles explains in the Pop Mech feature, “America Tried to Test a Massive New Weapon. It Turned Into a Nuclear Nightmare.”

“At the time, weapons scientists had calculated that the fireball the weapon created would stabilize above the stratosphere, where it would circulate and diffuse over a wide area, not create much immediate local fallout. Officials thought winds would take 12 to 15 hours to carry fallout to the inhabited atolls, by which point the material wouldn’t be so dangerous, having chilled and decayed.

‘It turns out that that just isn’t how it works,’ says Alex Wellerstein, the nuclear historian.

The blast disintegrated coral into the size of sand grains—perfect fallout, Wellerstein said, noting that the particles were bigger and heavier than the experimenters had anticipated. Larger particles fell faster than anticipated and landed closer to the detonation site. And they were more radioactive than expected because they had less time to decay in the atmosphere.”

Just hours after the blast, people on Rongelap watched a bizarre white powder drift down over their island. It landed on their houses, water supplies, food, skin, and hair. Children touched it and played in it.

The powder started as material from the blast site. When a nuclear fireball erupts close to coral reef and seawater, it can pull pulverized coral, sand, water, and bomb debris into the mushroom cloud. Those particles become contaminated with radioactive material, then fall back downwind. At Castle Bravo, the particles were larger and heavier than expected, so they dropped sooner and closer to the test site, while they were still extremely radioactive.

Two days later, residents of Rongelap and nearby affected areas were evacuated to Kwajalein Atoll for medical care and to prevent further exposure. But it was too late. By then, the fallout had already entered daily life. A medical report later found that exposed groups had received significant whole-body radiation, extensive skin contamination, and possible internal contamination from radioactive material. People exposed on Rongelap later reported symptoms including itching and burning of the skin and eyes, nausea, vomiting, and diarrhea, while skin lesions and hair loss appeared in the weeks that followed.

U.S. medical researchers studied the exposed people under Project 4.1, a program that provided care but also treated the accident as a rare chance to measure how radioactive fallout affected human bodies. According to the National Security Archive, Project 4.1’s directive warned personnel not to discuss the project’s purpose, background, or findings except with people who had a “need to know”.

The Castle Bravo disaster didn’t end there. Twenty-three crew members aboard the Japanese fishing boat Lucky Dragon No. 5 were also exposed, and one crewman later died. And the National Cancer Institute later estimated that as much as 1.6 percent of all cancers among Marshallese people alive between 1948 and 1970 might be attributable to fallout from U.S. nuclear testing, with the projected share rising to 55 percent among the 82 people exposed on Rongelap and nearby Ailinginae.

Lawrence Livermore National Laboratory’s Marshall Islands Program says the Rongelap community returned home in 1957, then left again in 1985 after Marshallese-language reports showed the atoll remained contaminated with long-lived radioactive material.

For the haunting full story of how Castle Bravo became America’s worst radiological disaster—from the lithium-7 mistake inside the bomb to the evacuation delays, the exposed Japanese fishing crew, the secrecy, and the decades of contamination that followed—read “America Tried to Test a Massive New Weapon. It Turned Into a Nuclear Nightmare.”