The missing thermonuclear weapon, which weighted 3.4 tonnes and was nearly 100 times more powerful than the bomb that destroyed Hiroshima, was jettisoned out of the bomb bay of a B-47 bomber. And, like Iraq's fabled WMDs, this weapon has never been found.

A half-century after being lost, the US-made bomb apparently lies on the ocean floor, a mere 40km from the city of Savannah.

This real-life nuclear nightmare began on a chilly February night in 1958, as Major Howard Richardson and his crew of two were on a flight 36,000 feet over rural Georgia. The Strategic Air Command (SAC) bomber had been in the air for eight hours, and had travelled 8,000km as part of a routine two-plane practice mission designed to simulate the rigors of a long distance attack mission to Moscow - should the cold war ever turn hot.

US fighter planes would often strafe the big bombers in simulated attacks. The bomber crews were aware of such training practices, but would never know where or when they might appear.

Some time after midnight, just as the two B-47 crews were beginning to relax, knowing they would only be strapped inside their cramped cockpits for another few hours, Lieutenant Clarence Stewart, flying an F-86 Sabre jet fighter lifted off from Charleston Air Force Base in South Carolina. Directed by ground control, he and two other fighter pilots rapidly caught up to the slower bombers.

But for some reason, never fully explained, neither ground control nor Lieutenant Stewart's own radar detected that there were not one, but two bombers in the air.

As he moved into position for a mock attack, he was unaware that he was descending directly on to a second bomber, that of Major Richardson. With an enormous flash of light, the two planes collided. The impact tore off one wing of the fighter plane, then the other. Lieutenant Stewart, just 23 years old, immediately pulled the handle on his ejector seat and was jettisoned, 12,000 metres above the Earth. He spent 30 minutes breathing through an oxygen bottle, before landing in what he later called 'the biggest damn swamp in South Carolina'. Several hours later, after repeatedly firing his flare gun, he was rescued by a forest ranger.

Meanwhile, the Boeing bomber was severely damaged, but flyable. The problem was that it must land in bad condition, with a nuclear weapon on board. Major Richardson, a veteran with 35 combat missions over Europe, cut the fuel to one bad engine and lowered his wheels to see if he could land safely.

But with one engine out and a smashed-up tail, the pilot decided not to take the risk. He radioed SAC headquarters, telling them that he would drop the bomb in the Atlantic Ocean, in shallow waters several kilometres off Tybee Island where he believed it would be quickly recovered. Major Richardson then skilfully landed his damage bomber and received the Distinguished Flying Cross as a result.

The US government launched a search covering 8sqkm of open waters off Tybee Island. For nine weeks, the island was closed to the public. Finally - as reported in a declassified 1958 memo - the Pentagon informed the Atomic Energy Commission: 'The search for this weapon was discontinued on 4-16-58. The weapon is considered irretrievably lost.'

For 47 years, there was no talk about the missing bomb. Then last autumn, the air force returned to Tybee Island after retired colonel Derek Duke, a local resident, measured unusually high radiation readings during his own private search of the waters off Tybee. Mr Duke said that even if the bomb wasn't set to detonate, it could still contain radioactive material.

A 20-member team of government scientists then spent nine months analysing radiation readings and underwater soil samples from the area. Their final report, issued a week ago, concluded that the bomb could not explode and should be left at sea. 'We still think it's irretrievably lost. We don't know where to look for it,' said Billy Mullins, the air force nuclear weapons adviser who led the second search in 50 years. 'You don't just send a guy down with a scuba tank and a shovel.'