At 6:45 a.m. 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 4½ miles high over Bikini Atoll in the Marshall Islands. The explosion, the U.S. government's first weaponized hydrogen bomb, was 1,000 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 7,000 square miles away.

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

It was part of a series of nuclear tests conducted as the U.S. military lurched into the nuclear age. From 1946 to 1958, 67 U.S. nuclear tests pulverized the reefs and islands of South Pacific.

U.N. Secretary-General Antonio Guterres, on a tour of the South Pacific to discuss climate change, said the U.S. built "a kind of coffin" 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 vulnerable. A strong storm could breach the dome, releasing the deadly legacy of the U.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," Guterres said, Agence France-Presse reported.

Guterres's "coffin" was the product of a belated U.S. response to the testing of the 1940s and 1950s. Beginning in 1977, the Defense Nuclear Agency began a sustained cleanup of the nuclear debris left on Enewetak Atoll, whose inhabitants were forced to relocate before the testing began. Beginning in 1977, 4,000 U.S. servicemen began collecting an estimated 73,000 cubic meters of tainted surface soil across the islands, the Marshall Islands' government said.

The material was transported to Runit Island, where a 328-foot crater remained from a 1958 test. For three years, the U.S. military dumped the material into the crater. Locals took to calling it "The Tomb," the Guardian reported.

In 1980, a concrete dome — 18 inches thick — was placed over the fallout debris. But the $218 million project was only supposed to be temporary until a more permanent site was developed, the Guardian said. However, no further plans were ever hatched.

In 1983, the Marshall Islands signed a compact with the U.S., 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.

According to a 2017 report by the Australian Broadcasting Corporation, among the fallout material was plutonium-239, one of the world's most toxic substances, with a radioactive half-life of 24,100 years. The staying power of that material is the problem. "That dome is the connection between the nuclear age and the climate change age," climate change activist Alson Kelen said.

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," said Michael Gerrard of Columbia University's Earth Institute. "It's permeable soil. There was no effort to line it. And therefore, the seawater is inside the dome."