The United States abandoned a nuclear weapon beneath the ice in northern Greenland following a crash in 1968, a BBC investigation has found.
Its unique vantage point - perched at the top of the world - has meant that Thule Air Base has been of immense strategic importance to the US since it was built in the early 1950s, allowing a radar to scan the skies for missiles coming over the North Pole.
The Pentagon believed the Soviet Union would take out the base as a prelude to a nuclear strike against the US and so in 1960 began flying “Chrome Dome” missions. Nuclear-armed B52 bombers continuously circled over Thule - and could head straight to Moscow if they witnessed its destruction.
But on 21 January 1968, one of those missions went wrong.