In 1968 the US government had been flying over Thule air force base for 8 years. The base was believed to be the first place Russia would attack and with US planes flying over it continually the US would know immediately if something happened to it. But it gets worse. The planes were all equipped with nuclear weapons, so that if Thule was destroyed they could make a beeline to Russia and blow the place up. This constant flying over Thule was part of a US operation called Chrome Dome.
In January 21, 1968 a B-52 was making its rounds near Thule and some foam cushions, which were brilliantly left on a heater, caught fire. The fire grew quickly and the crew had to abandon the plane before they could make their emergency landing. As with most of these broken arrow incidents, the nuclear part of the bomb did not go off, but instead the smaller explosions associated with the bombs did. This caused a lot of radioactive material to be spread across the snow in Greenland, where Thule base is located.
The contaminated snow was brought back the US. As were three of the four bombs. They had trouble locating the fourth. Seems it when right through the snow and down to the bottom of the ocean. Even after sending a submarine in to try to locate the bomb, the US was unable to retrieve it. So, it was abandoned there at the bottom of the ocean. I hope it is still there.