Researchers say they’ve resolved the 150-year mystery of what happened to crewmembers on the famous Civil War submarine H.L. Hunley, the first ever to sink an enemy warship during combat.

The eight Confederate crewmen were killed by the blast pressure from their own torpedo, which they set off on February 17, 1864 in a battle with the USS Housatonic. The explosion just 16 feet from the Hunley’s bow sunk the Housatonic,but also caused the submarine’s hull to flex and transmit a secondary, deadly shock wave inside the crew compartment, according to researchers at Duke University and elsewhere who conducted the study.

Their findings were published in PLOS One. 

The fate of the submarine and its crewmembers after the sinking of the Housatonic ranked as one of the greatest maritime mysteries. The submarine was raised off the ocean floor off Charleston, S.C. in 2000. The skeletal remains of the crewmembers were found, the men positioned at their battle stations.

Archeologists speculated that the men died when the Hunley was swamped or struck by a Union vessel, or that the submarine dove deep to avoid detection and never made it back to the surface.

Since no evidence was found that the men struggled to escape the submarine or to avoid drowning, one of the most popular theories was that a concussion from the torpedo explosion was to blame.

The Hunley now sits in a 75,000-gallon conservation tank at the Clemson Restoration Institute in North Charleston, S.C., where tourists can view the restoration process.

Conservators have been periodically chiseling away at encrusted sand, sediment and shells from the submarine to preserve the vessel, learn more about its design, operation and history.

The narrow, tapered submarine, 40-feet long and with a maximum width of just 4 feet, was fashioned from the wrought iron boiler of an older ship. The vessel’s commander could see out of a conning tower in the fore of the submarine. The remaining crewmen powered the vessel’s propeller using a hand crank.

During the battle, the submarine dragged a hinged, 16-foot spar with a 135-pound torpedo filled with black powder and fitted with a pressure-sensitive trigger bolted to its end.

When the ship was retrieved, the fore and aft conning towers – the only means of escape – were closed; the bilge pumps were not set to pump out water; keel ballast weights were firmly attached; and the crewmembers were found seated at their stations. In 2013, researchers established that the submarine was still attached to the explosive charge when it went off.

To better understand what happened to the Hunley, the Duke researchers built a 1/6 scale model of the submarine constructed of a 16-guage mild steel similar to that of the original submarine hull.

To gauge the impact of the explosion, they subjected the model, nicknamed CSS Tiny, to a series of increasingly large underwater blasts using different methods, so that the degree of pressure transmission to the scale model hull would be similar to the degree of pressure transmission into the full-sized Hunley.

They recreated black powder for the Hunley torpedo matching samples of powder from recently uncovered Union cannonballs from the Civil War.

The researchers concluded that the transmitted peak pressures transmitted to the Hunley were sufficient to cause a high risk of injury and fatality to the crew.

People exposed to such explosions are not always thrown violently, as often depicted in movies and on television, they noted. Blasts too weak to move a human body may still cause lethal pulmonary trauma.

Even if some crewmen survived the initial blast, researchers concluded, they likely would have been crippled in terms of respiration and physically unable to power the hand crank to move the submarine.

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