On April 10, 1963, the nuclear-powered killer sub Thresher, on a test dive off the Maine Continental shelf, encountered an as yet unspecified disaster and sank with 12 officers, 96 enlisted men and 21 "observers" aboard. It was the worst submarine loss in history. Even today the Navy keeps under wraps much so-called classified information that might lead to some kind of voracious explanation of the grim mystery.