The H.L. Hunley was the first submarine to sink an enemy warship. On the evening on February 17, 1864 during the American Civil War, this two-bit contraption fired a torpedo against the hull of the Union ship USS Housatonic in the waters off the coast of South Carolina, sinking it within minutes. Five of the 155 souls onboard died.

The eight Confederate soldiers crammed inside the submarine, however, were not seen again until they were raised from the seabed in 2000 by marine archaeologists. Bizarrely, they were still at their stations, and their skeletons had no obvious physical injuries, leading to a huge amount of speculation about what happened to the crew on that night over 150 years ago. One leading theory had been that a that a “lucky shot” from a crewmember onboard the Housatonic pierced the vessel and drowned the Confederates soldiers.

Now scientists have reinvestigated the sub’s mysterious demise and found the cause of death of those unfortunate sub-mariners. In a study in the journal PLOS One, they explain how the crew died from an intense blast of air caused by setting off the black powder torpedo.

"The disappearance of the Hunley has long stood as one of the great mysteries of American history," lead study author Rachel Lance of Duke University said in a statement. "Finding the cause of death of the crew has finally allowed us to declare the mystery solved."

A team of naval warfare experts and biomedical scientists reconstructed a scale model of the submarine using a similar steel used in Civil War-era ships. They mimicked the gunpowder explosions and blasted the submarine with pressurized air while monitoring the internal pressure inside the cabin generated by each blast.

They concluded these rushes of air to be sufficient enough to cause pulmonary and brain trauma, killing the crew instantly while barely leaving a broken bone. So, the crew was effectively killed by their own weapon.

“This is the characteristic trauma of blast victims, they call it ‘blast lung,’” Lance added in a statement from Duke University. “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.”

This was one of the earliest successful ventures into using submarines for warfare, although people had been toying with the idea since the days of ancient Greece. Like many early attempts, Hunley was highly precarious. On top of the vessel killing its own crew members, over 13 other people drowned during the testing and development of it.