Black seabirds circle high above the giant concrete dome that rises from a tangle of green vines just a few paces from the lapping waves of the Pacific. Half buried in the sand, the vast structure looks like a downed UFO. At the summit, figures carved into the weathered concrete state only the year of construction: 1979. Officially, this vast structure is known as the Runit Dome. Locals call it The Tomb. Below the 18-inch concrete cap rests the United States’ cold war legacy to this remote corner of the Pacific Ocean: 111,000 cubic yards of radioactive debris left behind after 12 years of nuclear tests. Brackish water pools around the edge of the dome, where sections of concrete have started to crack away. Underground, radioactive waste has already started to leach out of the crater: according to a 2013 report by the US Department of Energy, soil around the dome is already more contaminated than its contents. Now locals, scientists and environmental activists fear that a storm surge, typhoon or other cataclysmic event brought on by climate change could tear the concrete mantel wide open, releasing its contents into the Pacific Ocean. “Runit Dome represents a tragic confluence of nuclear testing and climate change,” said Michael Gerrard, director of the Sabin Center for Climate Change Law at Columbia University, who visited the dome in 2010. “It resulted from US nuclear testing and the leaving behind of large quantities of plutonium,” he said. “Now it has been gradually submerged as result of sea level rise from greenhouse gas emissions by industrial countries led by the United States.” Enewetak Atoll, and the much better-known Bikini Atoll, were the main sites of the United States Pacific Proving Grounds, the setting for dozens of atomic explosions during the early years of the cold war. The remote islands – roughly halfway between Australia and Hawaii – were deemed sufficiently distant from major population centres and shipping lanes, and in 1948, the local population of Micronesian fishermen and subsistence farmers were evacuated to another atoll 200 km away. In total, 67 nuclear and atmospheric bombs were detonated on Enewetak and Bikini between 1946 and 1958 – an explosive yield equivalent to 1.6 Hiroshima bombs detonated every day over the course of 12 years. The detonations blanketed the islands with irradiated debris, including Plutonium-239, the fissile isotope used in nuclear warheads, which has a half-life of 24,000 years. When the testing came to an end, the US Defence Nuclear Agency (DNA – later the DoE) carried out an eight-year cleanup, but Congress refused to fund a comprehensive decontamination programme to make the entire atoll fit for human settlement again. The DNA’s preferred option – deep ocean dumping – was prohibited by international treaties and hazardous waste regulations, and there was little appetite for transporting the irradiated refuse back to the US. In the end, US servicemen simply scraped off the islands’ contaminated topsoil and mixed it with radioactive debris. The resulting radioactive slurry was then dumped in an unlined 350-foot crater on Runit Island’s northern tip, and sealed under 358 concrete panels. But the dome was never meant to last. According to the World Health Organization, the $218m plan was designed as temporary fix: a way to store contaminated material until a permanent decontamination plan was devised. Meanwhile, only three of the atoll’s 40 islands were cleaned up, but not Enjebi, where half of Enewetak’s population had traditionally lived. And as costs spiralled, resettlement efforts of the northern part of the atoll stalled indefinitely. Nevertheless, in 1980, as the Americans prepared their own departure, the dri-Enewetak (“people of Enewetak”) were allowed to return to the atoll after 33 years. Three years later, the Marshall Islands signed a compact of free association with the US, granting its people certain privileges, but not full citizenship. The deal also settled of “all claims, past, present and future” related to the US Nuclear Testing Program – and left the Runit Dome under the responsibility of the Marshallese government. Today, the US government insists that it has honoured all its obligations, and that the jurisdiction for the dome and its toxic contents lies with the Marshall Islands. The Marshallese, meanwhile, say that a country with a population of 53,000 people and a GDP of $190m – most of it from US aid programs – is simply incapable of dealing with the potential radioactive catastrophe left behind by the Americans.