SOON AFTER THE NEWS about the stricken sailors broke, Yasushi Nishiwaki, a biophysics professor at Osaka City Medical University, received an urgent call from the local health department—part of the Lucky Dragon’s catch had already made its way to Osaka. The scientist grabbed his radiation meter and met authorities at the city’s bustling fish market.

Running the instrument over the tuna’s silver carcasses, Nishiwaki was shocked: a normal fish might cause 20 or 30 counts per minute (cpm), reflecting normal background radiation, but readings from the trawler’s tuna were as high as 2,000; in one case, 60,000 cpm was registered, according to former Manhattan Project physicist Ralph Lapp, who wrote a book about the Lucky Dragon incident.

Nishiwaki took an overnight train to Yaizu to inspect the moored ship. He measured high levels of radioactivity onboard and collected samples of remnant dust. Then he headed to Yaizu’s hospital and ran the counter over the fishermen, alarmed to discover they too were highly radioactive.

Nishiwaki, who just three years before had studied radiochemistry at Columbia University, implored the AEC for help: “To minimize possible radiation injury and damage to [the Japanese fishermen] we need to know immediately in detail the possible types of radioactive elements contained in the radioactive contaminated material.” He never heard back. Officials also ignored pleas from his American-born wife and collaborator, Jane, dismissing the couple as alarmists and Communist sympathizers.

While Nishiwaki collected data in Yaizu, health authorities swooped into markets countrywide to chase down and bury the Lucky Dragon’s contaminated catch. But hundreds of pounds had already been sold. Warnings not to consume the tuna hit the airwaves.

“Japan Alarmed By ‘Atomic Fish,’ ” announced front pages from Australia to Arkansas. “H-Bomb Burnings Blamed on Secrecy.” The media onslaught turned the clandestine weapon into the talk of the planet. Geiger counter sales skyrocketed as fear and outrage echoed around the globe. Indian Prime Minister Jawaharlal Nehru demanded a “standstill agreement” on testing.

“Everybody seems to think that we’re skunks, saber-rattlers, and warmongers,” President Dwight Eisenhower lamented to his National Security Council as the truth of the AEC’s staggering blunder leaked out.

WITHIN SECONDS, the scientists overseeing Castle Bravo realized they had woefully miscalculated the yield of their 23,500-pound device, codenamed Shrimp. The 15-megaton detonation—three times more powerful than what they had predicted and 1,000 times bigger than the blast that leveled Hiroshima—spewed radioactivity far wider than anticipated. In the Marshall Islands, 264 people were hospitalized, including 28 U.S. service members.

“Obviously, we didn’t know what the hell we were doing,” later remarked physicist Ted Taylor, who had designed nuclear bombs at Los Alamos. The size of the blast was “so far beyond predictions,” declared Chet Holifield, vice chair of the congressional Joint Committee on Atomic Energy, “you might say it was out of control.”

Going into the test, says nuclear historian Alex Wellerstein, the United States “had a very incorrect model of what was going to happen with fallout from multi-ton weapons.” The AEC believed “the mushroom cloud would be so hot and so large it would push above the stratosphere and get all the radioactive byproducts stuck up there.”

The particles, they thought, “would eventually come down, but they would have weeks and months to dilute, and the most radioactive parts would burn themselves out.” In reality, “the fallout from the Castle Bravo shot [shows] their model of stratospheric fallout is completely incorrect. This is dumping a lot of radioactivity, just fantastic amounts, directly downwind.”

However, the AEC wouldn’t cop to it. In mid-March, U.S. news wires reported the contents of letters home from a U.S. marine describing the evacuation of a group of badly burned islanders soon after the test. The AEC flatly denied the marine’s assertion that islanders had been hurt. “They were subjected to a certain amount of radiation,” said a congressman on the AEC oversight committee. “Everybody in the program is subjected to radiation.”

When Tokyo doctors diagnosed the Lucky Dragon crew as suffering from acute radiation sickness, U.S. officials scoffed at the diagnosis. Their burns weren’t caused by radioactivity, AEC director Lewis Strauss suggested, but by caustic coral from the vaporized isle. Radioactivity had been confined to within five miles of the test site, Strauss claimed, adding that the Japanese ship must have sailed into a “danger area” planners had mapped out around the test site that covered more than 50,000 square miles.

In fact, the ship had been 14 miles outside the zone’s border. The Japanese fishermen, the director assured, weren’t even very sick. “The blood count of these men is comparable to that of our weather station personnel who are fit to return to duty.” In truth, the AEC and State Department had begun discussing payoffs for the Lucky Dragon crew just three days after they reached port.

Privately, Strauss told Eisenhower officials he suspected the Lucky Dragon was a ruse—a “Red spy outfit” sent by Moscow to monitor the test—and directed the CIA to investigate if they were spying. W. Sterling Cole, head of the Joint Committee on Atomic Energy, made similar accusations publicly.

These claims inflamed the Japanese press, which was tracking the hospitalized fishermen’s worsening illness in daily reports. The Lucky Dragon incident had already deepened public distrust of the United States, which just nine years before had killed more than 100,000 with the atomic bombings of Hiroshima and Nagasaki by U.S. estimates (later estimates would more than double that tally).

After Japan’s surrender, media reports related to the bombings were heavily censored, and Japanese scientists were banned from researching or publishing on the bombing’s effects until U.S. occupation ended in April 1952. For the many Japanese who had only recently learned the details of the devastation in Hiroshima and Nagasaki, the fishermen’s saga touched a fresh wound. As U.S. ambassador John Allison noted in a March 21 telegram from Tokyo, many Japanese wondered why they “continue to be the world’s only victims of A-bombs.”

Absent help from the AEC, Allison reported, Japanese scientists had begun analyzing the “ashes of death” on their own. They found numerous fission byproducts and other radioisotopes, including uranium-237, in the dust they analyzed. Scientists relayed these findings to the Japanese press. Wild speculation followed, including the possibility that the Americans had detonated a bomb meant to spread radiation.

Now it was the AEC officials who were upset at the Japanese scientists for blabbing to the papers. But both groups soon found themselves pulled into a deepening crisis. A full-scale panic was rattling Japan’s fish markets, one starting to reverberate around the world.

JAPANESE CITY DWELLERS didn’t need to read the news to know something was wrong.

The country’s huge fish markets, normally jammed with shoppers, were empty. After news of the Lucky Dragon’s radioactive tuna broke, prices plummeted, but not as much as demand. Vendors posted “We don’t sell radioactive fish” signs above their stalls, but most people were unconvinced. Things got worse when reports circulated that the emperor had stopped eating seafood. The stench of rotting fish hung over cities like a miasma. It was a stunning shift for a nation that relied on seafood as a main source of protein.

On March 24, the U.S. embassy in Tokyo announced that “there is negligible hazard, if any, in the consumption of fish caught in the Pacific Ocean outside of the immediate test area.” It added that “any radioactivity collected in the test area would become harmless within a few miles after being picked up by [ocean] currents and completely undetectable within 500 miles or less.” Strauss maintained that, apart from the Lucky Dragon’s catch, no fish had been affected by U.S. testing.

Despite those assurances, radioactive tuna—along with radioactive sailors and ships—kept showing up at Japanese docks. And while U.S. officials insisted fish from the Pacific were absolutely safe, the FDA began surveying radiation levels of tuna imports from Japan, setting a standard of 100 cpm. At that news, the Japanese tuna market bottomed out.

The Japanese Ministry of Health and Welfare responded by making fishermen who had been anywhere near the U.S. testing area submit to Geiger checks of their boats, crews, and fish. The AEC recommended the agency adopt FDA standards for consistency; the health ministry complied, announcing that any fish exceeding 100 cpm should be disposed of at sea.

AEC Health and Safety Chief Merril Eisenbud traveled to Japan on March 22. Hearing reports that fishermen were dumping radioactive catches as instructed, Eisenbud suggested they were actually disposing of fish that had gone bad during the voyage. The AEC scientist refused to believe fish were exceeding 100 cpm. “I didn’t see [one] radioactive fish,” he said of his two-week visit.

Ambassador Allison later summarized the American perspective in a telegrammed report. Shocking revelations about the injured fishermen, he wrote, froze Japan’s fledgling, post-occupation government and pushed public anxiety of atomic weapons over the edge: “Period of uncontrolled masochism ensued, as nation aided by unscrupulous press, seemed to revel in fancied martyrdom.”

Allison also faulted “uncontrolled statements” by Japanese scientists and physicians, “many of whom were fuzzy-minded leftists, pacifists, neutralists. Nearly all seemed animated by resentments arising from occupation with its ban on Japanese atomic research.”

But fears of a contaminated food supply were not the product of mass psychosis and grievance Allison made them out to be. Boats were undoubtedly hauling in radioactive fish. This was confirmed by the FDA, which discovered several contaminated imports at California ports.

U.S. officials clamped down on that news, fearing a stateside panic like the one that had struck Japan. But the number of radioactive shipments kept growing as the U.S. military kept blasting thermonuclear devices at Bikini. By the end of June, hundreds of vessels had set off the Geiger counters when they arrived at Japan’s ports and more than 400,000 pounds of tuna had been destroyed.

The AEC hatched a solution: boost the permissible levels of radioactivity in fish. Japan’s scientists shot down the suggestion. Nevertheless, when Japanese researchers discovered radioactive rain was contaminating drinking water and being absorbed by vegetables and rice, the AEC officials tried the same approach, announcing levels of 5 microcuries per liter were permissible, even though such levels were 50,000 times higher than those recommended by their own scientific advisors.

In mid-May, soon after the U.S. military completed its sixth and final test in the Castle series, Japan’s Fisheries Agency sent 22 scientists on a two-month expedition to survey the waters between the Marshall Islands and Japan. The voyage was meant to bolster AEC assurances that radioactivity would disperse harmlessly within a few miles of the blast zone and so restore the public’s faith in the fish supply. What they found upended American assumptions.

Radioactivity did not dilute in the ocean, as the AEC predicted—quite the opposite. Radioactive isotopes, such as cesium-137 and strontium-90, concentrated in powerful currents—especially the North Equatorial Current, which flows northwest of the Marshall Islands. Readings of more than 7,000 cpm per liter were found 350 miles from the test site; other significantly high concentrations were detected thousands of miles from Bikini. More troubling, studies showed the radioactive isotopes concentrated in the food chain: they were taken up by plankton, which were devoured by smaller fish, which were eaten by large fish—including tuna—ending up in their organs.

The expedition, however, confirmed one finding that authorities could spin. Since most radioactivity was swept up in the northern currents, much lower readings were found in the waters to the south of the Marshall Islands—news the Fisheries Agency and U.S. press latched onto.

WHILE THE U.S. GOVERNMENT was skeptical of the Japanese findings, European scientists were starting to listen. In July, Nishiwaki set off for Western Europe to publicize the findings of the oceanic expedition, studies showing radioactivity was showing up in Japanese crops, and the strange isotopes Japanese scientists had discovered in dust that sickened the Lucky Dragon’s crew, who remained hospitalized.

In late September, the ship’s radioman, Aikichi Kuboyama, died. The New York Times declared him the “world’s first hydrogen-bomb casualty.” Japan went into mourning, while outrage and opposition to nuclear testing surged around the globe. Nishiwaki, still in Europe, was asked to explain Kuboyama’s death to BBC radio’s audience.

During the widely circulated broadcast, Nishiwaki noted that the crew had been sickened from both the inside and out—not only had the dust burned their skin, radioactivity had also infiltrated the men’s blood and organs when they breathed and swallowed it. He explained that high amounts of radioactivity were being found not only in Japan’s fish, but also its vegetables, milk, and in the lungs and livers of the country’s cows. He framed the communication of this research as a right and duty “so that other peoples will not suffer in the future.”

Perhaps most consequentially, during his travels Nishiwaki met Joseph Rotblat, a Polish physicist living in London who had the distinction of being the sole scientist to walk away from the Manhattan Project during World War II. Like other scientists, Rotblat had a keen interest in the hydrogen bomb developed by his former Manhattan Project colleague Edward Teller. (“It’s unreasonable to make such a big deal over the death of a fisherman,” Teller reportedly said after Kuboyama’s death.)

When Nishiwaki gave Rotblat papers showing the contents and radioactivity levels of the fallout that dusted the Lucky Dragon, Rotblat was astonished. Nishiwaki’s findings didn’t match up with the model of a hydrogen bomb promoted by advocates—a weapon touted as far more powerful yet far cleaner than the bombs dropped on Japan in 1945. “When I looked at the data given to me by my Japanese colleague,” Rotblat would later say, “I came to the conclusion that the bomb was a very dirty bomb.”

Rotblat correctly deciphered the device’s mechanics: a fission bomb ignited the hydrogen bomb at the core, which caused another fission reaction when neutrons bombarded a uranium shell. As Rotblat noted in an article published in the Atomic Scientists Journal, Castle Bravo’s “so-called hydrogen bomb is in reality a fission-fusion-fission bomb.”

Philosopher Bertrand Russell was so horrified when Rotblat told him of the findings that he broadcast a special Christmastime appeal on the BBC. “Is our race so destitute of wisdom, so incapable of impartial love, so blind even to the simplest dictates of self-preservation, that the last proof of its silly cleverness is to be the extermination of all life on our planet?” he asked.

Russell petitioned scientists to speak out against the danger the hydrogen bomb posed. Albert Einstein, in one of the last acts before his death, endorsed what became known as the Russell-Einstein Manifesto, which called on scientists to halt the progress of this new weapon. Humanity’s “continued existence,” the manifesto asserted, “is in doubt.”

Nishiwaki returned in November to a country brimming with angst and packing theaters to see filmmaker Ishirō Honda’s new hit. Opening with a ship destroyed by a mysterious blast, Godzilla cast the nation’s anxiety onto the big screen—the story of a sea monster awakened by nuclear testing and scientists’ struggle to understand and contain it. By December, around a million pounds of contaminated fish had been dumped and Japan was still awaiting compensation for injuries to its fisheries and the Lucky Dragon crew that Ambassador Allison had dangled since March.

Instead, the United States continued to pressure Japan to loosen its radioactivity standards—introducing the idea of “permissible doses” and recommending that the acceptable levels in tuna be changed to 500 cpm. To the outrage of most scientists, the Japanese government caved. Officials announced at the end of December that, based on these new standards, fish didn’t need to be monitored at all and ordered local governments to stop. The market bounced back.

“The contamination still continued, of course,” said historian Higuchi.

Two weeks later, the United States paid Japan $2 million—without admitting liability—to offset losses to the fishing industry and to compensate the Lucky Dragon’s crew.

According to fisherman Ōishi Matashichi, who would develop liver cancer and whose first child was stillborn, 16 out of 23 crew members died in their 40s or 50s. Matashichi became a nuclear disarmament advocate, often lecturing alongside the Lucky Dragon 5—made into a Tokyo museum in 1976—until his death in 2021.

Nishiwaki, for his part, continued tracking American contamination in Japanese fields and bodies. Across the Pacific, U.S. researchers had been following his lead.

The radioactivity Castle Bravo shot into the atmosphere on March 1, 1954, traveled the planet for months. By that November, said John Bugher, director of the AEC’s Division of Biology and Medicine, radioisotope iodine-131 was being “detected in thyroids all over the U.S.” According to calculations offered by AEC Commissioner Willard Libby, a 20-fold increase in radiation levels in the United States “was expected to show up in vegetation and food by Thanksgiving.”

In fact, as the Department of Commerce Weather Bureau noted the next year, apart from the tropics, more fallout from the Castle tests fell on the southwestern United States than anywhere else in the world—five times more than had fallen on Japan.