The first submarine to down an enemy ship was sunk itself after its crew failed to release an emergency weight to help it resurface.
Crew aboard the Confederate vessel HL Hunley did not disconnect the 1,000lb keel blocks to help it rapidly resurface, resulting in the sub being trapped underwater and the men dying from lack of oxygen.
Scientists who removed the corrosion, silt and shells from the boat found the levers all locked in their regular position, solving a mystery dating back to 1864. The blocks would typically keep the sub upright, but also could be released with three levers.
That would allow it to surface rapidly, archaeologist Michael Scafuri, who has worked on the submarine for 18 years, said.
'It's more evidence there wasn't much of a panic on board,' Scafuri said. The Hunley and its eight crewmembers disappeared in February 1864 in Charleston Harbor shortly after signaling it had placed explosives on the hull of the Union ship the USS Housatonic.
The Hunley had delivered a blast from 135 pounds of black powder below the waterline at the stern of the Housatonic, sinking the Union ship in less than five minutes.
Housatonic lost five seamen, but came to rest upright in 30 feet of water, which allowed the remaining crew to be rescued after climbing the rigging and deploying lifeboats. Ever since the Hunley was raised from the ocean floor in 2000, scientists have worked to determine why the sub never returned to the surface.