The incident came amid the height of the Cold War, when American B52 bombers were flying continuously around the country’s Thule Air Base to keep watch for possible missile strikes from the USSR.
Pentagon chiefs believed that the Soviet Union would seek to eradicate the base, whose ability to scan surrounding skies made it strategically crucial. They also feared a strike on the base could be a prelude to an attack on the US mainland.
Unbeknown to Denmark – of which Greenland is a self-governing province – the B52s were carrying nuclear bombs, in anticipation of flying direct to Moscow if any Soviet missiles destroyed the base.
And on January 21, 1968, one of the B52s crashed into ice a few miles away from the base, scattering potentially catastrophic debris. The high explosives surrounding the weapons were detonated, but the weapons themselves were not, as they required arming by the crew.
Following the crash, and a painstaking clear-up mission to recover thousands of pieces of material from 500 million gallons of ice, the Pentagon stated that all four weapons onboard had been “destroyed”.
However, documents obtained by the BBC under the US Freedom of Information act have disclosed that while this is technically true – none of the bombs remained complete – one of the weapons was not recovered.
As well as fears over its constituent radioactive uranium and plutonium, US authorities were highly concerned about a specimen nuclear bomb falling into enemy hands and disclosing the design secrets of its weaponry.
The secret reports disclose that by the end of January, officials were concerned about a dark patch of ice, and wrote: “Speculate something melted through ice, such as burning primary or secondary” parts of the weapon.
Three months later, the US sent a Star III submarine to the base to search for the bomb – and disguised the mission to the Danish authorities as a routine inspection of the crash site, the documents show.
However, the search was eventually abandoned, with the documents explaining that officials said it would not be possible to effectively search the entire affected area.
William H Chambers, a former designer of nuclear weapons who ran the accidents team that dealt with the Thule crash, told the BBC: “There was disappointment in what you might call a failure to return all of the components.”
However, he maintained that the US had concluded the risk posed to its security by abandoning the search was not great. He said they had believed “it would be very difficult for anyone else to recover classified pieces if we couldn’t find them.”