The US abandoned a nuclear weapon beneath the ice in northern Greenland following a crash in 1968, a BBC investigation has found.
Mystery of lost US nuclear bomb
The B-52 is a long-range, heavy bomber
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.
Greenland is a self-governing province of Denmark but the carrying of nuclear weapons over Danish territory was kept secret.
But on 21 January 1968, one of those missions went wrong.
BBC reunited two of the pilots, John Haug and Joe D'Amario, 40 years on to tell the story of how their plane ended up crashing on the ice a few miles out from the base.
In the aftermath, military personnel, local Greenlanders and Danish workers rushed to the scene to help.
Eventually, a remarkable operation would unfold over the coming months to recover thousands of tiny pieces of debris scattered across the frozen bay, as well as to collect some 500 million gallons of ice, some of it containing radioactive debris.
A declassified US government video, obtained by the BBC, documents the clear-up and gives some ideas of the scale of the operation.
The high explosives surrounding the four nuclear weapons had detonated but without setting off the actual nuclear devices, which had not been armed by the crew.
The Pentagon maintained that all four weapons had been "destroyed".
This may be technically true, since the bombs were no longer complete, but declassified documents obtained by the BBC under the US Freedom of Information Act, parts of which remain classified, reveal a much darker story, which has been confirmed by individuals involved in the clear-up and those who have had access to details since.
The documents make clear that within weeks of the incident, investigators piecing together the fragments realised that only three of the weapons could be accounted for.
Even by the end of January, one document talks of a blackened section of ice which had re-frozen with shroud lines from a weapon parachute. "Speculate something melted through ice such as burning primary or secondary," the document reads, the primary or secondary referring to parts of the weapon.
By April, a decision had been taken to send a Star III submarine to the base to look for the lost bomb, which had the serial number 78252. (A similar submarine search off the coast of Spain two years earlier had led to another weapon being recovered.)
But the real purpose of this search was deliberately hidden from Danish officials.
One document from July reads: "Fact that this operation includes search for object or missing weapon part is to be treated as confidential NOFORN", the last word meaning not to be disclosed to any foreign country.
"For discussion with Danes, this operation should be referred to as a survey repeat survey of bottom under impact point," it continued
The US was flying so-called Chrome Dome missions over Greenland
But the underwater search was beset by technical problems and, as winter encroached and the ice began to freeze over, the documents recount something approaching panic setting in.
As well as the fact they contained uranium and plutonium, the abandoned weapons parts were highly sensitive because of the way in which the design, shape and amount of uranium revealed classified elements of nuclear warhead design.
But eventually, the search was abandoned. Diagrams and notes included in the declassified documents make clear it was not possible to search the entire area where debris from the crash had spread.
We tracked down a number of officials who were involved in dealing with the aftermath of the incident.
One was William H Chambers, a former nuclear weapons designer at the Los Alamos nuclear laboratory who once ran a team dealing with accidents, including the Thule crash.
"There was disappointment in what you might call a failure to return all of the components," he told the BBC, explaining the logic behind the decision to abandon the search.
"It would be very difficult for anyone else to recover classified pieces if we couldn't find them."
The view was that no-one else would be able covertly to acquire the sensitive pieces and that the radioactive material would dissolve in such a large body of water, making it harmless.
Other officials who have seen classified files on the accident confirmed the abandonment of a weapon.
The Pentagon declined to comment on the investigation, referring back to previous official studies of the incident.
But the crash, clear-up and mystery of the lost bomb have continued to haunt those involved at the time - and those who live in the region now - with continued concerns over the environmental and health impact of the events of that day in 1968.