(CNN) -- Born and built amid gray-cloaked secrecy during the American Civil War, the H.L. Hunley -- the first submarine to sink an enemy ship -- has held tight to its murky mysteries.

The 150th anniversary of the Hunley's daring and dangerous raid will be marked this weekend and Monday, but the overarching question remains: What caused the submarine and its eight-member crew to slip to the bottom of the sea on the moonlit evening of February 17, 1864, after it signaled to shore a success that changed naval warfare.

The Hunley, housed at a laboratory in North Charleston, South Carolina, has yielded its secrets slowly and sparingly, even to researchers armed with the latest in technology....