The US military set off its largest nuclear explosion in testing 69 years ago, but scientists had no idea the blast would be that big

• Dozens of nuclear tests were carried out by the US in the Pacific between 1946 and 1958.
• The largest of these was the detonation of the Castle Bravo device on March 1, 1954. 
• It was 1,000 times more powerful than the atomic bomb that leveled Hiroshima during World War II. 

The US military tested dozens of nuclear weapons in the Pacific in the years following World War II, but none of the blasts compared in size to that of the Castle Bravo test — what a nuclear weapons historian once called "the greatest single radiological disaster in American history."

The Castle Bravo device was the largest nuclear weapon ever tested by the US. On March 1, 1954 — exactly 69 years ago Wednesday — the military detonated this bomb at Bikini Atoll, a small coral reef in the Marshall Islands. 

The radioactive fallout produced by the blast was much worse than anyone anticipated and was an eyeopening incident that raised questions about the development of atmospheric testing, a nuclear weapons expert told Insider. 

Castle Bravo was the first of six weapons to be tested as part of a weeks-long series experimenting with large-yield thermonuclear devices — also known as Operation Castle — that stretched from March 1 to May 31, 1954. The 23,500-pound device caused a mushroom cloud that reached an altitude of 130,000 feet and left a crater that was 250 feet deep and over 6,500 feet wide.   

But scientists severely miscalculated the yield, predicting it would only produce an explosive yield of at most six megatons, according to the Brookings Institute. They didn't realize the fusion fuel would have so much of an impact on the bomb's detonation.  

The detonation produced an actual explosive yield of 15 megatons, the Department of Defense said in an Operation Castle fact sheet, meaning it was 1,000 times more powerful than the atomic bomb that flattened Hiroshima, Japan, at the end of World War II. 

'It got a lot worse'

Castle Bravo also caused a significant amount of fallout that had negative implications for years. Radioactive material fell on residents of the Marshall Islands and was even found in communities thousands of miles away, leading to severe illnesses, and American service members tasked with cleaning up the nuclear tests were exposed to contaminated food and dust, which caused lasting health issues. 

A Japanese fishing vessel that was sailing around 80 miles away from the test site on the day the Castle Bravo device was detonated was exposed to radiation that eventually killed one sailor and left others on board with radiation poisoning, according to the Atomic Heritage Foundation. And nearly 40 US Navy sailors exposed to the fallout suffered skin lesions, the Department of Defense noted.  

Whenever the military readied for a nuclear test, it set up an safety zone in preparation for the radioactive fallout that ensued after the device was detonated. But when a bomb produces a greater explosive yield than anticipated, these prior assumptions about safety can be compromised, Hans Kristensen, the director of the nuclear information project at the Federation of American Scientists, told Insider.  

"What happened after Bravo, of course, was that a lot of the radioactive fallout was greatly amplified and drifted over a much larger area than it was anticipated," Kristensen said. "Fallout was expected to be significant in any case. But it got a lot worse because of that." 

The fallout led to serious health effects among the residents of the Marshall Islands, although its unclear exactly how many people were impacted by testing. The National Cancer Institute found that some cancers in residents who were alive during the years-long period of nuclear testing may be attributed to radiation exposure. Castle Bravo alone was responsible for nearly all of the radiation doses in the country's northern atolls.  

A 2019 report by the Republic of the Marshall Islands' National Nuclear Commission said nuclear weapons testing — like Castle Bravo — had a catastrophic impact on the country's environment, and some individuals still fear on a daily basis how their long-term exposure to the radiation might impact their health. 

"There were real implications for people living on surrounding islands," Kristensen said. "And those consequences have been very severe" and long lasting, he added.

Castle Bravo was a real 'eyeopener'

Despite the devastation caused by Castle Bravo, the US military continued to conducting nuclear weapons testing in the Pacific. Between 1946 and 1958, a total of 67 tests were conducted. 

But the incident helped the US — and global community — to realize the grave dangers associated with this particular type of testing, as continued atomic and hydrogen bomb detonations led to increasing concerns about the impact of radioactive fallout. 

One year later, in 1955, the United Nations established the Scientific Committee on the Effects of Atomic Radiation, which was tasked with monitoring the impact of radiation exposure and helping governments and organizations craft protective measures and evaluate the risk of radiation. 

The US, UK, and Soviet Union signed the Limited Test Ban Treaty in 1963, which barred nuclear weapons testing in the atmosphere, in outer space, and underwater. The agreement was the first of several that targeted nuclear weapons during the Cold War era but did not prohibit underground testing. The follow-on Comprehensive Nuclear-Test-Ban Treaty was signed in the 1990s.

"It definitely contributed to this realization that atmospheric nuclear testing is perhaps not so smart," Kristensen explained, adding that "this was a real sort of eyeopener, I think, for the global community also. I mean, it was so powerful and devastating that it really helped push that."