Despite heightened concern since Sept. 11, the U.S. Air Force said it is not losing sleep over a nuclear bomb lost off the coast of Georgia during the height of the Cold War.

The nuke was lost during a 1958 training exercise, when an F-86 fighter plane collided with a B-47 bomber. The pilot of the crippled B-47 released a "Mark-15" bomb over water to reduce the risk of an explosion during the emergency landing.

A nine-week search following the accident found no sign of the missing bomb, and the Air Force deemed the Mark-15 "irretrievably lost."

For more than four decades, the bomb has presumably remained embedded somewhere in the seabed — invisible to the world and, for the most part, ignored.

But Derek Duke, a retired Air Force lieutenant colonel, fears the bomb may fall into the wrong hands and wants the government to renew efforts to find it.

"This bomb is about 100 times more powerful than the Hiroshima bomb," Duke said.

That would be the case only if the bomb was armed with a plutonium capsule necessary for creating a thermonuclear reaction. The Air Force claims no plutonium was installed on the bomb, since it was being used only in an exercise.

"This is a simulated weapon. It’s not a full bomb," said Maj. Donald Robbins, a nuclear weapons expert with the Air Force.

Duke and others who want to find the bomb point to a 1966 Department of Defense document in which W.J. Howard, then assistant secretary of defense, described the bomb as a "complete weapon."

But Air Force officials said the document was in error and the government later corrected it. Duke said the discrepancy leads him to suspect a cover-up.

Duke said new technology would increase the government’s chances of finding and recovering the bomb. But the Air Force maintains that such an operation could cost taxpayers more than $5 million and might be more dangerous than leaving the bomb alone.

While denying the existence of nuclear materials inside the bomb, Air Force officials said the Mark-15 contained approximately 400 pounds of conventional explosives, which could blow up if disturbed.

An Air Force study released in April 2001 concluded, "There could be substantial economic impact to the region if an accidental detonation of the conventional explosive occurred during search or recovery operations."

The bomb is believed to be buried 5 to 15 feet under the seabed off the Georgia coast, in an area frequented by fishing vessels and recreational boaters.

"It is in the best interest of the public and the environment to leave the bomb in its resting place and remain categorized as irretrievably lost," the Air Force study said.