At 6.45am on March 1, 1954, the blue sky stretching over the south Pacific Ocean was split open by an enormous red flash.

Within seconds, a mushroom cloud towered 7km-high over Bikini Atoll in the Marshall Islands.

The explosion, the US Government's first weaponised hydrogen bomb, was 1000 times more powerful than the "Little Boy" atomic bomb blast that flattened Hiroshima - and a complete miscalculation.

Scientists had underestimated the size of what became known as the "Castle Bravo" test, resulting in an explosion that was two-and-a-half times larger than expected. Radioactive ash dropped more than 18,130 sq km from the bomb site, caking the nearby inhabited islands.

"Within hours, the atoll was covered with a fine, white, powder-like substance," the Marshall Islands health minister would later testify, according to the Atomic Heritage Foundation. "No one knew it was radioactive fallout. The children played in the 'snow.' They ate it."

The 1954 explosion was part of a series of nuclear tests conducted as the American military lurched into the nuclear age. From 1946 to 1958, 67 US nuclear tests pulverised the tranquil reefs and islands of South Pacific. International pressure finally halted the bombing, but the damage was done - and continues to this day.

That was the message reiterated by UN Secretary-General António Guterres on a recent tour of the South Pacific to discuss climate change.

In Fiji last week, he told the crowd about "a kind of coffin" built by the US in the Marshall Islands to house the deadly radioactive debris from 1980s. The structure, however, was never meant to last. Today, due to disrepair and rising sea tides, it is dangerously vulnerable. A strong storm could breach the dome, releasing the deadly legacy of America's nuclear might.

"I've just been with the President of the Marshall Islands (Hilda Heine), who is very worried because there is a risk of leaking of radioactive materials that are contained in a kind of coffin in the area," Guterres said in Fiji, AFP reported.

Guterres's "coffin" was the product of a belated American response to the testing of the 1940s and 1950s. Beginning in 1977, the Defence Nuclear Agency began a sustained cleanup of the nuclear debris left over on Enewetak Atoll, a slender archipelago in the Marshall Islands's northwest corner.

Enewetak Atoll was subjected to repeated blasts during the testing, and inhabitants were forced to relocate before the explosions began. Beginning in 1977, 4000 US servicemen began collecting an estimated 73,000 cu m of tainted surface soil across the islands, according to the Marshall Islands' Government.

The material was then transported to Runit Island, where a 100m crater remained from a May 1958 test explosion. For three years, the American military dumped the material into the crater. Six men reportedly died during the work. Locals took to calling it "The Tomb," the Guardian reported.

In 1980, a massive concrete dome - 45cm-thick and shaped like a flying saucer - was placed over the fallout debris, sealing off the material on Runit. But the US$218 million project was only supposed to be temporary until a more permanent site was developed, according to the Guardian. However, no further plans were ever hatched.

In 1983, the Marshall Islands signed a compact of free association with the US, granting the island nation the right to govern itself. But the deal also settled "all claims, past, present and future" tied to the nuclear testing, and left the dome in the care of the island government.

According to a 2017 report by the Australian Broadcasting Corporation, among the fallout material was plutonium-239, an isotope that is one of the world's most toxic substances, and one with a radioactive half-life of 24,100 years.

The staying power of that material is the problem. It's still there, only 45cm of concrete away from waters that are rising.

"That dome is the connection between the nuclear age and the climate change age," climate change activist Alson Kelen told the Australian broadcaster.

Cracks reportedly have started to appear in the dome. Part of the threat is that the crater was never properly lined, meaning rising seawater could breach the structural integrity.

"The bottom of the dome is just what was left behind by the nuclear weapons explosion," Michael Gerrard, the chair of Columbia University's Earth Institute, told the ABC. "It's permeable soil. There was no effort to line it. And therefore, the seawater is inside the dome."

According to the Guardian, a 2013 report by the Energy Department admitted radioactive material may have already begun to leak from the dome, but cautioned the health risks were likely low.

The Marshallese Government, however, does not have the money to shore up the structure, leaving it vulnerable to both rising tides and typhoons.

"It's clear as day that the local government will neither have the expertise or funds to fix the problem if it needs a particular fix," a Marshallese official told the Guardian.

Last week, Guterres sounded a similar theme in Fiji about the ongoing effects of the American testing on the small island nation.

"The Pacific was victimised in the past as we all know," he said, according to AFP. "The consequences of these have been quite dramatic, in relation to health, in relation to the poisoning of waters in some areas."