Just after midnight on Feb. 5, 1958, two U.S. Air Force jets, each traveling 500 mph, collided 35,000 feet over the Georgia countryside. Improbably, all four crew members survived and the accident might have passed into dim memory if not for the thermonuclear weapon jettisoned off Tybee Island, Ga. The bomb is still there. After a weeks-long search, it was declared irretrievably lost on 16 April 1958," the Air Force reported four years ago in an assessment of whether to conduct a new search and recovery mission. It concluded that "it is in the best interest of the public and the environment to leave the bomb in its resting-place." The Navy Supervisor of Salvage, the report noted, didn't think the bomb could be found. Energy Department engineers' best guess was that it lay "buried nose-down, probably 5-15 feet below the seabed." Clearly, the Air Force would have been glad to let it go at that. However, it did not count on the determination of Derek Duke, a 60-year-old retired Air Force officer who lives nearby and for more than six years has been searching for the bomb in the waters around Tybee Island, about 16 miles from Savannah. Responding to Duke's claim that he had found an area of high radiation the Air Force returned last September to look again. The report on the new search has not been released. Any danger still presented by the Tybee bomb is from the 400 pounds of conventional explosives or from humans somehow ingesting uranium that might escape from the bomb and its silt prison.