The atoll was picked as the location for Operation Crossroads, a program to investigate the effects of nuclear blasts on Navy vessels. On July 1, 1946, Test Able was staged. A target fleet of 95 ships was positioned in Bikini Atoll’s lagoon, with laboratory animals—pigs, goats and mice—on board so that scientists could study the potential effects of radiation on ship crews. A support fleet of another 150 ships withdrew to a position 10 nautical miles from the Atoll, and waited.

At 9 a.m., a B-29 bomber flew over the lagoon and dropped an atomic bomb, which exploded 520 feet from the surface and missed the target ship in the middle of the lagoon by 1,500 to 2,000 feet, according to an account from the Atomic Heritage Foundation. The bomb only sunk five of the ships, but the force of the blast and radiation killed about a third of the lab animals.

In Test Baker on July 25, 1946, the U.S. military tried a different approach, exploding a bomb 90 feet beneath the water surface of the lagoon. It was the first underwater test of a nuclear weapon, and resulted in all sorts of startling phenomena, according to the Atomic Heritage Foundation. The blast generated a massive bubble of hot gas that simultaneously expanded downward and upward.

At the bottom, it carved a 30-foot-deep, 2,000-foot-wide crater in the surface of the sea floor. On the surface, it burst through like a geyser and created an enormous dome of water that eventually reached more than a mile in height. The blast triggered a tsunami with a 94-foot-high wave, so powerful that it lifted up the Arkansas, a 27,000-ton ship. The surge of water swept over many of the target ships, coating them with radioactivity. Eight of the ships were sunk, according to a U.S. Navy account.

The U.S. allowed international observers at the tests, and Lavrentiy Pavlovich Beria, who was both head of the Soviet atomic program and chief of the Stalin regime’s secret police, sent a physicist and a geologist, according to Richard Rhodes’ 1995 book Dark Sun: The Making of the Hydrogen Bomb.

Apparently, they weren’t impressed. One of the observers Simon Peter Alexandrov, who was in charge of uranium for the Soviet’s own nuclear effort, told a U.S. scientist there that if the purpose of the test was to frighten the Soviets, it hadn’t worked, because the Soviets had bombers that could reach U.S. cities, according to the National Security Archive. The Soviet newspaper Pravda subsequently criticized the U.S. tests as “common blackmail” and said that other than a few obsolete warships, the only thing the United States had blown up was “belief in the seriousness of American talk about atomic disarmament.”

The U.S. nuclear weapons program had a problem in 1946, because it didn’t yet have that many bombs. The Able and Baker tests used up two of the only three nuclear cores in the U.S. stockpile, according to Rhodes. Even though production of new bombs soon picked up, the U.S. military remained concerned about squandering resources. Operation Crossroads originally was to have included a third test, Charlie, scheduled for April 1947, in which researchers planned to explode an atomic bomb even deeper in the water. But senior officials at the Manhattan Project and the Pentagon argued that it had no military value, and that providing another bomb would hinder the efforts to produce a lighter and smaller atomic weapon, according to the National Security Archive’s account. The test was postponed and eventually canceled. Officials apparently were also unhappy with the atoll’s lack of land to create a support base and the inability to build an airstrip there. After the 1946 tests, Bikini Atoll wasn’t used again as a site until 1954, when the U.S. began to test hydrogen bombs.

The Bravo test wasn’t the first H-bomb that the US. detonated—that distinction belonged to Ivy Mike, a device exploded in November 1952 in the Enewak Atoll in the Marshall Islands. But it was the first thermonuclear weapon that was small enough to be utilized as a weapon. While its designers had achieved a technological first, they also made a critical mistake, by drastically underestimating the size of the yield that would be created by its fusion fuel.

When the 23,500-pound device was detonated on March 1, 1954, it produced a 15-megaton blast—three times as big as planned, according to a Brookings Institution report. The explosion was so powerful that it vaporized three of the islands in the atoll, and tore a mile-wide crater in the bottom of the lagoon.

Stanford University biology professor Stephen Palumbi, who visited the atoll in 2017 as part of a TV documentary, estimated that the bomb blast hurled debris in the air that was the equivalent of 216 Empire State Buildings, according to Stanford Magazine.

The radioactive debris spewed by the blast contaminated 23 crew members aboard a Japanese fishing boat located 80 miles away, as well as residents of Rongelap and Utirik atolls. Kuboyama Aikichi, a crew member from the Japanese boat died six months later at age 40. Japanese physicians who performed an autopsy on Aikichi cited radiation sickness as the cause of death, although that determination remained disputed.

The Bravo test’s nuclear device was nicknamed “Shrimp,” even though it weighed 23,500 pounds. The Romeo test, conducted a few weeks after Bravo, used an even bigger bomb dubbed “Runt I.” Other bombs had nicknames such as “Morgenstern” and “Alarm Clock,” according to the NRDC report.

When the Bikini Atoll’s inhabitants were relocated in 1946, it was promised that they eventually could return. Instead, they were relocated to other islands in the Marshalls. Starting in the late 1960s, the U.S. Atomic Energy Commission declared Bikini Atoll finally to be safe again for human habitation, and allowed some former residents to return. But that experiment was cut short a decade later, when a study showed that the levels of Cesium-137 in returnees’ bodies had increased by 75 percent.

The Bikini inhabitants were relocated once again, this time to Kili Island, 450 miles away. Scientists say it’s still not safe to return. “Probably the most robust finding from our research is that Bikini Island must be cleaned up if people are to live there again,” says Ivana Nikolic Hughes, a senior lecturer in chemistry at Columbia University and Director of the K-1 Project Center for Nuclear Studies “This is based on levels of Cesium-137 in the food, background gamma radiation, and presence of various isotopes in soil and ocean sediment.”

In 2010, UNESCO declared Bikini Atoll a World Heritage Site as a reminder of the fearsome power of nuclear weapons and their influence on modern civilization.