Project Crested Ice
January 21, 1968 - This was the day a United States Air Force B-52 crashed onto the sea ice just off Greenland. The bomber was carrying four hydrogen bombs and they ruptured on impact scattering radioactive debris across miles of sea ice. One of the bombs was never completely accounted for.
The crash was caused by a fire in the cockpit that forced the crew to bail out. 6 men ejected safely, but one was killed. The burning plane continued to fly for a while before disintegrating across the ice.
In addition to the 225,000 pounds of aviation fuel, there were high levels of deadly plutonium, uranium, tritium and americium spread across a vast area.
Since Greenland is a Danish territory, the US and Denmark initiated a joint cleanup operation called Project Crested Ice. For the next 9 months, crews scraped the contaminated ice and placed over 550,000 gallons of radioactive liquid and debris into steel containers that were shipped back to America for disposal.
The Pentagon claimed that all four hydrogen bombs were destroyed in the crash, but subsequent searches with Navy submarines indicate they were still looking for something down there.
In the aftermath of the disaster, the USAF discontinued (or more likely scaled back) its Cold War program of keeping nuclear-armed bombers aloft at all times. Time magazine listed the Thule crash as one of the ten worst nuclear disasters in history.
Here's an image of the crash site: