Foreign Correspondent's Mark Willacy visited the Marshall Islands and discovered more about its dark legacy.
Some 23 atomic bomb tests were carried out on the Bikini Atoll and at least 40 more were done at Enewetak atoll, which includes Runit, and other Marshalls atolls in the 1940s and 1950s.
Speaking to news.com.au ahead of the program going to air Monday night, Willacy said those who remained living in the region were worried.
The reporter said visiting Runit was eerie and it was at first hard to believe toxic material lay underneath an island with an azure lagoon and green scrub.
"It's only from the air that you get to see how big it really is," he said.
Runit Island, located on the remote Enewetak Atoll, was the scene of the biggest nuclear clean-up in US history.
Willacy said rising sea levels meant water has begun to penetrate the dome containing the toxic waste with radioactive material leaking out.
A 2013 report commissioned by the US Department of Energy confirmed the dome was leaking.
While the US paid for the clean-up, Willacy said initial plans to lining the bottom of the dome with concrete didn't go ahead and the soil was permeable, which meant seawater gets inside.
"The dome was only meant to be a temporary solution until the US came up with a permanent plan," he said. "Instead it was a shoddy cost cutting exercise."
Despite a $US2.3 billion compensation award, only $US4 million has been paid out.
He said cracks are visible in the dome's surface but said even if the structure failed the US government didn't necessarily believe it would lead to a change in the contamination levels in the waters surrounding it.
Residents on Enewetak Atoll fear they will have to move if the dome collapses completely and warn a leak would not only be devastating for them but for would cause a toxic nightmare for the whole Pacific.
"One woman I spoke to called it a tomb because it could be their graveyard," he said.
Willacy, who has covered the devastation of the 2011 Japanese tsunami and Fukushima disaster, said there were differences in how the Asian nation and the US dealt with the radiation clean up.
"Fukushima was heavily populated and Japan was at fault so had to deal with it, and couldn't just pour concrete on it and walk away," he said.
He said the Marshall Islands had a handful of villages in comparison and was much more remote.
Willacy said those living in the Marshall Islands remained concerned due to long term exposure effects and rising sea levels caused by climate change.
"Sea levels will have a lot more impact than just high end real estate being flooded," he said.
"More than 50,000 people live in the Marshall Islands and climate change is happening for these people now."
Marshallese community leader Alson Kelen told Willacy the dome was the connection between the nuclear age and the climate change age.
"We're not talking just the Marshall Islands, we're talking the whole Pacific Ocean" he said.
Nuclear disarmament campaigner John Hallam said the Marshall Islands were a US trust territory on which the US carried out their largest ever nuclear test, the Castle Bravo test, of 15 megaton yield, on March 1, 1954.
"The Castle Bravo test was itself much larger than had been planned and almost resulted in the destruction of the reinforced concrete control bunker, which started moving violently as the ground shock reached it," he said.
"It also resulted in massive fallout, some of which is underneath that concrete dome."
This test was 1000 times as powerful as the US nuclear bombs dropped on Hiroshima and Nagasaki in 1945.
Hallam said photos of the dome showed a pool of water directly under it and it came so no surprise that it's leaking.
"When the dome was constructed, the US DoD (Department of Defense) almost contemptuously reassured the RMI (Republic of Marshall Islands) government that it would last for the next 200,000 years. This is of course nonsense, and it's now breaking apart."
He said the Marshall Islands story is part of the wider one of nuclear testing in the pacific, carried out by the US, France, and the UK.
