The US Congress has demanded an investigation into a concrete dome full of nuclear waste, threatened by rising sea levels in the Marshall Islands.
Dubbed "the tomb", it holds tonnes of radioactive debris from dozens of US atomic bomb tests carried out during the Cold War.
Congress wants the Department of Energy to report back on the state of the ageing Runit Dome within six months.
The demand is part of a huge defence bill approved by President Trump.
More than 40 nuclear weapons tests took place on or near the Enewetak Atoll in the Pacific between 1946 and 1958, including a bomb test on Runit Island.
The crater from that blast was used from the 1970s by the Defense Nuclear Agency to store nuclear waste. It was later covered in thick concrete slabs to form the 377ft (115 metre) protective dome.
There are concerns that the structure is deteriorating and could be vulnerable to sea-level rise caused by climate change.
Energy Secretary Dan Brouillette is required to report back to the House and Senate Armed Services Committees with "a detailed plan to repair the dome to ensure that it does not have any harmful effects to the local population, environment or wildlife."
The report must also assess the condition of the outside of the structure and what effect the environment and rising sea levels might have.
The condition of the dome was highlighted in a recent Los Angeles Times investigation, which also unearthed evidence of biological weapons tests and the shipping of 130 tonnes of soil from nuclear testing grounds in Nevada to the Marshall Islands.
UN Secretary General Antonio Guterres expressed concern about the "risk of leaking of radioactive materials" from the dome after meeting Marshall Islands President Hilda Heine in May.
The amendment to the defence bill was brought forward by Democratic Congresswoman Tulsi Gabbard, who is in the race to become the Democrats' nominee in next year's presidential election and represents Hawaii's second district.
"The US government is responsible for this storage site and must ensure the protection of the people and our environment from the toxic waste stored there," she said in June.
At the time of nuclear tests, the Marshall Islands was a territory created by the UN but administered by the US. It has been independent since 1979.