A Civil War mystery that has held strong for over 130 years finally has an answer. The sinking of the combat submarine H.L. Hunley, resulting in the deaths of its eight crew members, has puzzled historians and naval researchers alike for over a lifetime, but a new research effort has led to a likely explanation: the submarine’s own weapon is what ultimately doomed its passengers.

The H.L. Hunley was a truly historic vessel. Built in the 1860s, the Confederate submarine became the very first to successfully sink an enemy ship when it attacked the USS Housatonic with a mine in February of 1864, sending the massive Union warship into the depths.

After completing its mission, the submarine was expected to return to port but never arrived. It was ultimately found over a century later, in 1995, on the sea floor near the wreckage of the Housatonic, with the remains of its eight crew members still positioned in front of their stations. Researcher Rachel Lance of Duke University believes she’s discovered what happened, and it all comes back to the massive barrel-like mine the sub used as a weapon.

The mine — a thick copper cylinder packed with over a hundred pounds of black powder  — was attached to the end of a long rigid pole on the front of submarine. The sub’s only method of attack was to ram the mine right into the side of its target, which is exactly what it did to the Housatonic.

Unfortunately for the crew, the underwater blast sent a shockwave through the water and the nearby submarine as well. The impact felt by the crew would have been several orders of magnitude greater than if they had been standing on dry land, and soft tissue like the brain and lungs would have been particularly vulnerable to the concussive force. The crew, it is thought, died instantly, which is why their bodies remain forever at their stations.

The sub itself was only mildly damaged from the incident, but without a functioning crew (and with a broken window to boot) it ultimately drifted to the sea floor below.

Mike Wehner has reported on technology and video games for the past decade, covering breaking news and trends in VR, wearables, smartphones, and future tech. Most recently, Mike served as Tech Editor at The Daily Dot, and has been featured in USA Today, Time.com, and countless other web and print outlets. His love of reporting is second only to his gaming addiction.