An oil painting by Conrad Wise Chapman, “Submarine Torpedo Boat H.L. Hunley, Dec. 6, 1863”

It was a technological marvel: the first submarine ever to sink an enemy ship. The H.L. Hunley entered legend when it sank the U.S.S. Housatonic off the coast of Charleston on Feb. 17, 1864, and then itself never made it back home to port. The mystique of the lost submarine only increased when it was discovered on the ocean floor in 1995 – with the skeletons of its crew still at their stations.

Now, a new comprehensive investigation has determined that it was the H.L. Hunley’s own weapon that killed the crew, and sent the first-of-its-kind vessel to the bottom for good, as presented in the journal PLOS One today.

The submarine had a long spar with a torpedo attached at its end containing black powder – which the Confederate crew used to spear and sink the Housatonic.

But the Duke University team investigating the Hunley – raised in 2000 – conducted a three-investigation study that concluded it was the very blast that sent the submarine into the history books and instantly condemned it to a watery grave.

The copper keg with gunpowder was secured on the end of the 16-foot pole. But the farthest any of the Hunley crew were from the blast was 42 feet - much closer than in any of the trials during the craft's development, the Duke team found.

The Duke team repeatedly conducted its own trials including blasts near a scale model, shooting authentic weapons at iron plates, and recalculating blast effects on the human body.

The result was that the sheer force of the underwater blast walloped the brains and lungs of the submarine’s crew, killing them instantly.

Without the hands at the crank to drive the ship onward, it gradually drifted out on the tide and took on water gradually before sinking to the bottom, about 300 meters away from its enemy, they concluded.

The forensic recreation of the blast, the deaths, and the scene within the narrow craft was done over a period of three years by a team led by Rachel M. Lance, a Duke researcher who is also a biomedical engineer for the U.S. Navy.

First, the skeletons showed no broken bones, and the positions of the bodies showed no signs of trying to escape. The conning towers were closed, and one of them was still locked. The bilge pumps were not set to pump out water, and the keel ballast weights used to keep the craft submerged had not been released to allow it to rise, the team reported.

“All the physical evidence points to the crew taking absolutely no action in response to a flood or loss of air,” said Lance.

Two large holes in the bow and the hull were determined to have been caused after the sinking, based on sediment layers within the ship – which showed the submarine had been sealed for a long period of time upon hitting bottom. The holes’ cause was also pinpointed to a combination of corrosion, stress on the riveted seams, and ocean-current erosion, they write.

But the shockwave reconstruction was another vital piece of evidence, they report. Using the one-sixth scale model of the Hunley (dubbed the C.S.S. Tiny), outfitted with sensors inside, they reconstructed what the estimated blast would have done to living beings sealed within the ship.

The traumatic energy would be enough to shear delicate structures like the lungs and brain – soft tissues that decomposed – while leaving the bones completely intact and still in their places on the 4-foot-wide tube-shaped ship.

“This is the characteristic trauma of blast victims, they call it ‘blast lung,’” said Lance. “You have an instant fatality that leaves no marks on the skeletal remains. Unfortunately, the soft tissues that would show us what happened have decomposed in the past hundred years.”

The Hunley may have been doomed from the start. Owing to the power of the gunpowder and the proximity to the blast, each of the eight crewmembers had less than 16 percent chance of survival, according to the team's calculations. Two sinkings had even occurred during testing and development of the submarine, killing 13 including the craft’s namesake, Horace L. Hunley.

Although eyewitnesses later reported seeing the blue light on the water – the submarine’s pre-arranged victory signal – the authors found that the “heat of battle” leading to “notoriously unreliable” reports.

The “Pyrrhic attack” of the Hunley, overall, killed 21 Confederates and only five Union sailors.