Runit Dome in the Marshall Islands is facing a problem that cannot be seen from above. Groundwater has been moving beneath the site for decades, while about 85,000 cubic meters of radioactive waste remain buried inside the old nuclear test crater. Now, rising Pacific sea levels are adding new pressure to the aging structure. The concern is not simply whether the concrete dome will crack. Scientists are watching how tides, groundwater and seawater movement could affect radioactive contamination beneath and around Runit Dome.
Rising Pacific Seas Threaten Radioactive Runit Dome in Marshall Islands
The story begins in 1958, when the United States detonated the 18-kiloton Cactus nuclear test on Runit Island, part of Enewetak Atoll. The explosion carved a crater roughly 30 feet deep and more than 100 meters across. Two decades later, that same crater became the disposal site for contaminated soil and debris collected during a massive cleanup operation across Enewetak.
Between 1977 and 1980, workers moved radioactive material into the crater and eventually covered it with a concrete cap. The structure became known as the Runit Dome, or simply “the Tomb.” Estimates put the volume of material inside at roughly 84,900 cubic meters, or about 111,000 cubic yards. The material contains long-lived radionuclides, including plutonium isotopes and americium, left behind by decades of nuclear weapons testing.
The important detail is what was not built beneath it. There is no impermeable liner separating the radioactive material from the geological structure below. The crater floor remains hydraulically connected with the surrounding environment, and fractures created by the original nuclear explosion provide pathways through the underlying rock. That makes the dome very different from a modern engineered radioactive waste repository designed to isolate contaminants from groundwater.
The ground beneath the Dome is not still
Enewetak is an atoll built largely from ancient coral material. Coral sediments and reef limestone contain an interconnected network of pores and fractures, allowing groundwater to move through the island. Because the islands sit only slightly above the Pacific, the boundary between freshwater, brackish groundwater and seawater is constantly shifting. Tides can therefore influence water movement below the surface.
This is the strange part of Runit Dome. A concrete cap can look solid from above while water continues moving underneath it. As the tide rises, seawater can push into the porous subsurface. As the tide falls, groundwater can move back toward the lagoon. Scientists describe this broader process as submarine groundwater discharge, and it can transport dissolved chemicals and fine particles from land into coastal waters.That does not mean the entire radioactive inventory is being flushed into the Pacific. Measurements have shown a much more complicated picture. Research published in 2018 found that Runit Dome was likely associated with about half of the plutonium in the Enewetak Lagoon water column based on plutonium isotope signatures, yet groundwater flowing directly from beneath the dome appeared to be only a small pathway for plutonium export. Much of the radioactive contamination remains in lagoon sediments.
Why Radioactive material can move with water
The behavior of radionuclides in groundwater depends on chemistry as much as it depends on the movement of water. Plutonium, for example, does not simply dissolve like salt and travel freely through every pore. Its mobility changes with oxidation state, acidity, dissolved organic material and interactions with minerals and particles. Some forms can attach strongly to sediments, while others can form soluble or colloidal compounds that move more readily.
The conditions beneath a buried waste site can also differ sharply from those at the surface. Researchers have identified low-oxygen conditions and chemical reactions that can influence how radionuclides bind to particles or enter groundwater. This matters because even a small amount of mobile contamination can migrate through a porous environment over long periods. The question is therefore not simply whether water enters the waste, but what that water picks up along the way.
There is another important distinction. Plutonium-239 has a half-life of about 24,100 years. That does not mean a radioactive release automatically becomes a catastrophic hazard, but it does mean the material remains part of the environment on a timescale vastly longer than the lifespan of the concrete structure containing it. The engineering problem is therefore measured in centuries and millennia, not decades.
Rising seas change the conditions around runit
Climate change introduces a second physical problem. Sea-level rise raises the baseline from which tides, waves and storm surges operate. On a low coral atoll, a relatively small change in average sea level can have an outsized effect because there is little land elevation separating groundwater, infrastructure and the ocean. Higher water levels can also increase the frequency with which coastal areas experience flooding.
For Runit Dome, that matters both above and below the concrete cap. More frequent seawater intrusion can alter groundwater movement beneath the structure, while stronger coastal flooding can increase erosion around the island. Extreme storms create another pathway for contamination by disturbing lagoon sediments that already contain radioactive material. Climate modeling for Enewetak has found that storms can substantially redistribute contaminated sediments under some scenarios.
A 2024 review in Communications Earth & Environment placed Runit Dome within a broader environmental problem: climate change can alter the release and movement of radionuclides from old contaminated sites. Sea-level rise, changing rainfall, warming and extreme weather can all change how contaminants move between land, groundwater and the ocean. The researchers stressed that these interactions need to be assessed rather than assuming that old contamination will remain physically stable forever.
Cracks have added a new warning sign
The concrete itself is now becoming part of the scientific concern. Reports and observations have documented cracking in the dome’s outer structure, while recent reporting in 2026 has again drawn attention to visible deterioration and groundwater moving through the base. Scientists who have examined the structure say its aging cannot be separated from the changing coastal environment around it.
That does not mean Runit Dome is on the verge of suddenly collapsing. The U.S. Department of Energy has said the observed cracking is consistent with aging concrete and has not identified an imminent collapse threat. Its monitoring program includes periodic visual inspections and radiochemical testing of groundwater around the structure. The latest available federal assessment found that several radionuclide concentrations in groundwater remained below drinking-water limits, although strontium-90 and gross beta activity were notable exceptions.
The more difficult scientific question is what happens over a much longer period. A dome can remain structurally adequate today while still becoming progressively less reliable as concrete ages, sea level rises and storms interact with a low-lying island. Climate change does not need to cause an immediate failure to matter. It can slowly change the hydraulic and physical conditions that determine how contaminants move.
The Lagoon already contains a nuclear legacy
Runit Dome is also only one piece of a much larger radioactive footprint. Between 1946 and 1958, the United States conducted dozens of nuclear tests across Bikini and Enewetak atolls. Radioactive fallout settled across islands, reef flats and lagoon sediments. Some of that contamination remains detectable today, decades after atmospheric testing ended.
This creates an uncomfortable scientific paradox. Measurements show that Runit is an important source associated with plutonium contamination, yet the sediments across the lagoon are themselves a major long-term reservoir. Researchers have found that the annual transfer of plutonium and cesium from the atolls to the wider North Pacific is relatively small compared with the enormous releases produced during the original nuclear testing era. The danger is therefore not best understood as a giant radioactive plume suddenly spreading across the ocean.
Instead, the concern is persistence. Radioactive particles can remain trapped in sediments, move during storms, enter groundwater, settle elsewhere and become available to marine ecosystems. A change in the physical environment can alter that balance. The science is less about one dramatic failure than about whether decades of slow movement become more significant as the surrounding ocean rises.
Runit Dome is becoming a climate science problem
In 2022, Congress directed the Department of Energy to examine how climate change could affect the Runit Dome site. The resulting federal work considered available data on radioactive contamination and climate-related hazards, including the potential consequences of changing environmental conditions. The assessment reflects a growing recognition that old nuclear waste sites cannot always be evaluated using the climate and coastal conditions that existed when they were constructed.
For the Marshall Islands, this is not an abstract engineering exercise. Enewetak is inhabited, and the lagoon remains important to local communities. The dome sits roughly 30 kilometers from communities that use the lagoon, according to recent reporting. Any future change in contamination levels therefore has to be considered alongside food sources, groundwater, fisheries and the practical ability of local authorities to monitor and manage the site.
The real mystery beneath Runit Dome is consequently not whether water is moving. Scientists already know that it is. The harder question is how that underground circulation will change as the Pacific rises, how much radioactive material it can mobilize, and whether the concrete structure can remain reliable long enough for the contamination inside it to become less of a threat. For a site built to contain the consequences of a 1958 nuclear blast, the next chapter may be shaped less by nuclear physics than by groundwater, tides, concrete and a changing ocean.
