According to CNN, the USAF and DOE are investigating the reported find of
the notorious Tybee Island Mk 15 that had been jettisoned by a B-47 after
its collision with a F-86 in 1958. A retired USAF LTC led the group that has
found what is described as a large metallic object with radioactive
emissons. More accurately, what was lost was the bomb casing--the "pit" had
not been installed in the weapon when it was dumped into the ocean.

http://www.cnn.com/2004/US/09/13/lost.bomb/