In 1977, US authorities put the most contaminated debris and soil into a huge concrete dome called the Runit Dome, which sits on Enewetak Atoll and houses 88,000 square metres of contaminated soil and debris.
It has recently received media attention as it appears to be leaking, due to cracking and the threat from rising sea levels, while some Marshallese have fears it may eventually collapse.
However, American officials have said it is not their problem and responsibility falls on the Marshallese, as it is their land.
The US has cited a 1986 compact of free association, which released the US government from further liability, which will go up for renegotiation in 2023.