The US buried 85,000 cubic metres of nuclear waste beneath a Pacific island. Now the ocean is rising around it | World News

On a remote island in the Marshall Islands, a concrete dome built more than four decades ago holds roughly 85,000 cubic metres of radioactive waste left over from the United States’ Cold War-era nuclear testing programme. Known officially as Runit Dome and locally as The Tomb, the structure was built between 1977 and 1980 to seal off contaminated soil and debris from 43 nuclear weapons tests carried out by the US military across Enewetak Atoll between 1948 and 1958. The dome’s concrete cap sits directly above a blast crater that was never lined underneath, and rising sea levels are now pushing seawater further into contact with the waste inside, prompting the US Congress to order a formal government study into the risks the site poses as the climate continues to change.

How Runit Dome was built and what it contains

Runit Dome sits on Runit Island, part of Enewetak Atoll, inside a crater created by an 18 kiloton nuclear test known as Cactus, carried out in 1958. Following the end of nuclear testing in the region, around 4,000 US servicemen worked between 1977 and 1980 to gather contaminated soil, military equipment and other radioactive debris scattered across the atoll and bury it inside this crater, which was then capped with an 18-inch-thick layer of concrete. Because the underlying crater was never lined at the bottom, the waste sits in direct contact with the surrounding porous coral and groundwater, which US officials at the time considered an acceptable risk given the site’s remote location.According to acongressional report published by the US Department of Energy, Congress directed the department in 2021 to study the impacts of climate change on the Runit Dome site, along with other environmental hazards resulting from the broader nuclear testing programme carried out across the Marshall Islands. The Department of Energy contracted the Pacific Northwest National Laboratory to lead this assessment, bringing together climate scientists, ocean modellers, environmental scientists and health physicists to study the site in detail.

What the government study found about rising sea levels

The resulting study modelled how rising sea levels and storm surges could affect the site both under current conditions and in projected conditions for the year 2090, including a scenario involving a hypothetical structural failure of the dome itself. According to the same Department of Energy report, researchers found that storm surge combined with gradual sea level rise would have the greatest influence on how radioactive material might move and spread across the atoll over time, more so than the dome’s physical integrity alone.Despite these concerns, the study’s overall findings offered some reassurance regarding immediate risk to nearby residents. The report concluded that current and projected future conditions, including a hypothetical dome failure, did not indicate an increased health risk to residents of inhabited islands within Enewetak Atoll, and that any changes in radiological exposure to marine life within the lagoon were not expected to meaningfully affect the health or diversity of the atoll’s ecosystem.

Why the dome’s location makes it especially vulnerable

Much of Runit Island sits only a few metres above sea level, leaving very little buffer between the structure and rising tides. Since the dome was completed in 1980, sea levels in the surrounding region have already risen measurably, and climate projections suggest this trend will continue well into the coming decades, gradually increasing the frequency and intensity of storm surges capable of reaching the site.The Department of Energy’s assessment specifically identified this combination of gradual sea level rise and episodic storm surges, rather than a single catastrophic collapse, as the primary long-term concern for the site. This distinction matters for how the risk is understood, since it points toward slow, cumulative environmental change as the main driver of potential future exposure, rather than a sudden structural failure of the dome itself.

What this means for the Marshall Islands going forward

The Runit Dome remains a deeply sensitive issue for the Marshall Islands, a nation whose communities were displaced and exposed to fallout during decades of US nuclear testing before the cleanup effort that created the dome even began. Even as the Department of Energy’s study found no immediate elevated health risk under current or modelled future conditions, the underlying reality remains that a large volume of radioactive material sits in an unlined crater on a low-lying island in a region facing among the most severe sea level rise projections in the world.As climate change continues to reshape coastlines across the Pacific, Runit Dome stands as a stark example of how decisions made decades ago, under very different environmental assumptions, are now being reassessed under conditions their original designers never fully anticipated. The Department of Energy’s ongoing monitoring and periodic climate impact assessments suggest that Runit Dome will likely remain the subject of continued scientific and political attention for years to come, particularly as sea level projections for the region continue to be refined.