As nuclear explosions go, the U.S. "Cactus" bomb test in May 1958 was relatively small -- but it has left a lasting legacy for the Marshall Islands in a dome-shaped radioactive dump.

The dome -- described by a U.N. chief Antonio Guterres as "a kind of coffin" -- was built two decades after the blast in the Pacific Ocean region.

The U.S. military filled the bomb crater on Runit island with radioactive waste, capped it with concrete, and told displaced residents of the Pacific's remote Enewetak atoll they could safely return home.

But Runit's 45-centimeter (18-inch) thick concrete dome has now developed cracks.

And because the 115-meter wide crater was never lined, there are fears radioactive contaminants are leaching through the island's porous coral rock into the ocean.

The concerns have intensified amid climate change. Rising seas, encroaching on the low-lying nation, are threatening to undermine the dome's structural integrity.

Jack Ading, who represents the area in the Marshalls' parliament, calls the dome a "monstrosity."

"It is stuffed with radioactive contaminants that include plutonium-239, one of the most toxic substances known to man," he told AFP.

"The coffin is leaking its poison into the surrounding environment. And to make matters even worse, we're told not to worry about this leakage because the radioactivity outside of the dome is at least as bad as the radioactivity inside of it."

- 'Staggering' challenges -

The dome has become a symbol of the mess left by the U.S. nuclear test program in the Marshall Islands when 67 bombs were detonated between 1947-58 at Enewetak and Bikini atolls.

Numerous islanders were forcibly evacuated from ancestral lands and resettled, including Enewetak's residents. Thousands of more islanders were exposed to radioactive fallout and suffered health problems.

The people of Enewetak were allowed home in 1980, and about 800 islanders now live in the southern part of the atoll, 20 kilometers (12 miles) from Runit.

After the U.S. military withdrew, the Marshall Islands government officially accepted a "full and final" settlement to cover the impact of the nuclear tests.

But there have long been complaints that the compensation paid by Washington was inadequate, and the United Nations has described "a legacy of distrust" toward the United States.

U.N. Secretary-General Guterres raised the issue earlier this month after meeting Marshall Islands President Hilda Heine in Fiji, when they discussed the nuclear legacy and the prospect of radioactive leakage from Runit dome.

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

Marshalls Foreign Minister John Silk said he appreciated Guterres bringing the Runit dome to world attention with this comments.

"We are pleased that the Secretary-General made these statements since so often it seems that these ongoing legacy issues that continue to impact our people are forgotten by the international community," he said.

Uncertain future

Rhea Moss-Christian, who chairs the Marshall Islands National Nuclear Commission, said the country "needs the support of the international community to address the staggering health and environmental challenges across the Pacific."

The consequences of the dome failing are unclear.

A 2013 inspection commissioned by the U.S. government suggested radioactive fallout in the Enewetak lagoon sediment was already so high a catastrophic failure would not necessarily result in locals receiving increased dosages of radiation.

Silk, noting that the U.S. government had committed to ongoing monitoring of the dome, said an independent assessment of the structure's status "would be helpful."

But Ading said the situation was "a constant source of anxiety for the people of Enewetak."

"We pray that the Runit dome does not eventually become our coffin," he said.