A four-megaton nuclear bomb was one switch away from exploding over the US in 1961, a newly declassified US document confirms.
Two bombs were on board a B-52 plane that went into an uncontrolled spin over North Carolina - both bombs fell and one began the detonation process.
The US government has acknowledged the accident before, but never made public how close the bomb came to detonating.
The plane was on a routine flight when it began to break up over North Carolina on 23 January 1961.
As it was breaking apart, a control inside the cockpit released the two Mark 39 hydrogen bombs over Goldsboro.
One fell to the ground unarmed. But the second "assumed it was being deliberately released over an enemy target - and went through all its arming mechanisms save one, and very nearly detonated over North Carolina,"
Only the failure of a single low-voltage switch prevented disaster
The bomb was almost 260 times powerful than the bombs that fell on Hiroshima and Nagasaki.
The accident occurred during the height of the Cold War between US and Russia, just over a year before the Cuban missile crisis brought nuclear fears to the US's front door.