Shipwreck disaster experts are calling for a deep-sea expedition to a lost U.S. nuclear attack sub, the USS Scorpion, in an effort to verify a new theory on what caused the Cold War vessel to sink.  The Scorpion was lost May 22, 1968, killing 99 men, about 400 miles south of the Azores Islands in the middle of the Atlantic Ocean. Evidence now is directed at the sub's propeller shaft.  Some suggest that rubber bearings holding the propeller shaft failed, putting stress on the coupling connecting it to the engine. The coupling's bolts failed catastrophically during a deep test dive, spilling water into the sub too rapidly to allow ballast maneuvers to raise the ship to the surface.