On Feb. 5, 1958, during a simulated combat mission near Savannah, Ga., a B-47 bomber collided with an F-86 fighter. The fighter crashed; the bomber, barely airworthy, needed to reduce weight to avoid an emergency landing.

So it dropped a 7,600-pound nuclear bomb.

The bomb contained 400 pounds of conventional explosives and highly enriched uranium. There’s some disagreement as to whether it included the plutonium capsule needed to start a nuclear reaction.

That’s rather important, because in 50 years of searching the Air Force still hasn’t found the bomb. It hit the water near Tybee Island off the Georgia coast and is presumably buried in 10 feet of silt somewhere in Wassaw Sound. But exactly where it is, and how dangerous it is, remain unknown.