Scientists have solved a longstanding mystery about the first submarine ever to sink an enemy ship — what killed the sub's own crew.

On Feb. 17, 1864, during the American Civil War, the 12-metre long Confederate submarine H.L. Hunley made history when its torpedo took down the 1,100-tonne Union ship USS Housatonic outside Charleston Harbor, S.C.

The Hunley itself later sank, with its crew of eight aboard.

According to research led by Rachel Lance, who studied the incident during her Ph.D. in biomedical engineering at Duke University, the crew were killed by massive lung and brain injuries caused indirectly by their own torpedo. Lance, who graduated in 2016, published the findings Wednesday in the journal PLOS ONE.

The sunken submarine was found in 1995 and raised from the bottom in 2000. Mysteriously, the skeletons of all eight of the crew were all still at their stations, with no broken bones, and the sub was in very good condition, Lance reports.

The sunken submarine was found in 1995 and raised from the bottom in 2000. It had been undamaged by the blast and the crew's skeletons were still at their stations. (Bruce Smith/Associated Press)

"There were some holes in the hull that were the result of time under the sea. But there was no actual damage found to have happened from the blast itself," she said in an interview with Duke University.

The exit hatches were closed and the bilge pumps that would have been used if the sub started to take on water were not set to pump, suggesting that the crew never tried to save themselves as the sub sank.

Still, some scientists had proposed that the crew may have suffocated or drowned.

Recreating the blast

Lance solved the mystery by creating a 2-metre-long scale model made of mild steel, fitting it with sensors, and setting off a series of blasts intended to recreate the torpedo explosion.

Unlike a modern-day torpedo, the Hunley's weapon couldn't be fired into the water and away from the sub. Instead, it was a copper keg of gunpowder attached in front of the sub by a short pole called a spar that was rammed into the enemy ship by the advancing sub, with the crew inside.

"Their spar was only 16 feet long, so they were actually very close to the 135 pound charge, especially since the spar was at a downward angle," Lance said.

The sub's torpedo was a copper keg of gunpowder attached in front of the sub by a short pole called a spar that was rammed into the enemy ship by the advancing sub, not far from the crew inside the sub's hull. (Lance et al.)

When the charge exploded, the blast would have caused the submarine's hull to transmit a powerful, secondary shock wave into the submarine, crushing their lungs and brain and killing them instantly. Lance calculated that each crew member had only a 15 per cent chance of survival from the blast.

In fact, there was no indication that any of them survived.

In the end, the crew of the USS Housatonic fared better. Five of its members died in the torpedo blast, but the damaged ship came to rest in relatively shallow water, allowing the survivors to climb rigging, deploy lifeboats and escape.

The research was funded by Duke University, the U.S. Department of Defence, the U.S. Army and the Hagley Library's Center for the History of Business, Technology and Society.