In the midst of the Cold War, the US build a military base under the ice in Greenland. They dug tunnels under the glacier that covers the island, thinking that the ice would be permanent. The ice wasn't permanent even back then. Glaciers shift, and that made the Camp Century unstable, and ultimately unusable. But that was after it had been in use for ten years.
By the time the base was abandoned in 1967, it had its own library and theater, an infirmary, kitchen and mess hall, a chapel, and two power plants (one nuclear, one run on diesel). When the base closed, key parts of the nuclear power plant were removed, but most of the base’s infrastructure was left behind—the buildings, the railways, the sewage, the diesel fuel, and the low-level radioactive waste. In the 2016 paper, which Colgan worked on as well, the researchers suggested that the radiological waste was less worrisome than the more extensive chemical waste, from diesel fuel and polychlorinated biphenyls (PCBs) used to insulate fluids and paints.
Overall, the researchers estimated that 20,000 liters of chemical waste remain at the Camp Century site, along with 24 million liters of “biological waste associated with untreated sewage.” That’s just at Camp Century; the military closed down bases at three other sites in Greenland, too, and it’s unclear how much waste is left there. Over the next few decades, the researchers found, melt water from the ice sheets could mobilize these pollutants, exposing both the wildlife and humans living in Greenland.
For 50 years, military officials assumed that the abandoned base would remain buried. But now that the Greenland ice sheet is receding under ever-warmer climate conditions, what's left behind could be an environmental disaster in the making. Read more about Camp Century at Atlas Obscura.
Greenland is making a lot of noise about this issue, because they want to get some money out of it, but the site and presumably the cleanup was a NATO contribution by a member state, while the US paid the bulk for the men and materials. Somebody agreed to share the check, and decades later changed their mind. They have infinitely more of a case against Denmark than the USA.