The United States abandoned a nuclear weapon beneath the ice in northern Greenland following a crash in 1968, a BBC investigation has found.
On 21 January 1968, one of the missions went wrong.
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".
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. 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.
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.
"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.