It was, at the time, the biggest bang humans had ever made. The US nuclear test at Bikini Atoll in the Marshall Islands on March 1, 1954, was 1000 times larger than the bombs that obliterated Hiroshima and Nagasaki.
That's not in the same league as the fury nature unleashed on Krakatoa or beneath what is now Lake Taupo, but the 15-megaton blast - codenamed Castle Bravo - has been exceeded artificially only once, by the Soviets' 50-plus megaton Tsar Bomba in 1961.
What happened next was one of the great nightmares of the nuclear age. Fallout from Castle Bravo drifted over the inhabited atolls of Rongelap, Rongerik and Utirik.
On Rongelap, children played in highly radioactive incinerated coral, thinking it was storybook snow.
An hour after the explosion, the per-hour radiation level on the islands was 130 roentgen (R); 50 hours on, it was 175R. Normal background exposure is about 20R in a lifetime.
More than 60 years on, the fallout, literal and moral, from the test has not been cleaned up.
Two generations of birth defects and cancers, notably thyroid cancers and leukaemia, have ravaged the atolls' population, and women have given birth to babies that looked like bunches of grapes or jellyfish.
The US Government has never denied the islands were contaminated. But eight months after Bravo, the word "accidental" began to appear in all official documents. It has never been removed.
Now, an independent American documentary film, Nuclear Savage, which will screen in Auckland next week and Wellington next month, gives for the first time solid documentary evidence of deliberation.
Adam Horowitz, whose 1990 film Home on the Range reported on the islanders' plight, is scathing about his Government's actions and subsequent inaction.
Speaking from his base in Santa Fe, New Mexico, he says the documents constitute "black and white" evidence of what has long been alleged and suspected - that the contamination of the Marshalls atolls, far from being an unhappy accident, was a premeditated, minutely planned and cynically executed experiment to establish the long-term effects of radiation poisoning on humans.
"A lot of people over the years have talked about experimentation, but we were always dealing with allegation and suspicion and assertion," he said. "There was no hard evidence of experimentation. Now there is."
The documents in the film show the existence, a year before the test, of a programme within Bravo, numbered 4.1, and labelled "a study of the response of human beings exposed to significant beta and gamma radiation due to fallout from high-yield weapons".
Dr Robert Conard examines Rongelapese boy Hiroshi Kebenli in March, 1954. Hiroshi died when he was a teenager
It is the first public evidence that the Americans expected - indeed planned for - contamination of human subjects, and it lends weight to the oft-repeated assertions that the test went ahead even though the weather forecast indicated a danger that the wind would blow fallout over Rongelap.
The official story is that the US immediately evacuated the islands - "as soon as it was apparent that these people were exposed to fallout", says a memo over the signature of Secretary of State John Foster Dulles.
But surviving islanders interviewed for the film say it was two days before they were evacuated.
In November 1957, the Americans returned the Rongelapese to their island, assuring them it was safe.
Yet on July 14, 1956, more than a year before the islanders went home, US Atomic Energy Commission official Merril Eisenbud wrote: "That island is by far the most contaminated place on earth and it will be very interesting to get a measure of human uptake when people live in a contaminated environment."
In another document, Dr Robert Conard, who headed the American programme to treat victims of radiation-related diseases, remarks that the Rongelapese "will afford most valuable ecological radiation data on human beings".
The revelations are a major development in a lamentably old story. For at least 30 years, the plight of the Marshall Islands guinea pigs has been detailed in hundreds of articles and official reports, in books and films and news reports.
Under the heading "Misery of nuclear refugees" in this newspaper in September 1986, I wrote about the last voyage the year before of the ill-fated Greenpeace vessel Rainbow Warrior to evacuate the 350 residents of the island of Rongelap "to escape the radioactive poison ... in every coconut fibre and every grain of sand".
Today, these nuclear refugees live in shacks on the island of Ebeye, 300km southwest of Rongelap, in a festering slum called "the ghetto of the Pacific" - 16,000 people in 36ha - and continue to lobby for restitution.
But the Americans regard the matter as done and dusted. In a 1986 treaty, the islands entered a Compact of Free Association" (CFA) with the US, which gave them $US150 million in compensation.
The treaty also changed the islands' legal status. No longer a United Nations territory administered by the United States, it was denied access to the American legalsystem.
The CFA agreement also established a nuclear claims tribunal, which in 2007 awarded more than $1.8 billion for the damage done to the islands and the poisoning of their inhabitants. The US continues to ignore that award, made by a tribunal it agreed to establish, insisting that the 1986 payment was for "all claims past, present and future".
"When they made the agreement for a full and final settlement, they knew about all these documents and they kept them secret," says Horowitz.
"The Marshall Islanders did not know the extent of the damage and the long-term consequences, much less the extent of the premeditation."
The words "past, present and future" are in the CFA agreement. But the landscape changed unrecognisably between 1986 and 2007.
In a Pulitzer Prize-winning series of reports in the Albuquerque Tribune in 1994, journalist Eileen Welsome revealed that during the Cold War, the US Government covertly conducted human radiation experiments on American civilians, including pregnant women and disabled orphan children.
In the ensuing uproar, the Clinton Administration ordered the release of more than a million classified documents. A wealth of material relating to Castle Bravo spilled out, too, and the Marshall Islands' long-time Minister of Foreign Affairs, Tony DeBrum, who has been tirelessly lobbying congressmen and bureaucrats since the late 1970s, started looking through it.
In the US, Horowitz says, the story remains marginalised. His film shows in festivals but the Public Broadcasting Service, which part-funded it and has broadcast rights, has repeatedly cancelled scheduled broadcasts and insisted on re-edits that cut it from 90 minutes to 55 and then to 30.
It has yet to air in the US and is not available online.
"Getting the story told and in the public view is hard," Horowitz says. "But it is an important story. There were thousands of Americans murdered by radiation experiments, and the Marshall Islands story is one of a list of deliberate exposures of people that were done during the Cold War."
The US Embassy in Wellington did not respond to written questions and requests for an interview.
• Nuclear Savage screens on May 22, 24 and 31 as part of the Documentary Edge Festival.