Modern nuclear submarines are some of the most advanced military machines ever created, representing the latest in a long line of technological evolution going back centuries. Believe it or not, some early submarines sailed the seas prior to the 1900s, and while rudimentary compared to today’s standard military machines, they’re significant to history. One of the most important subs of the 19th century was the H.L. Hunley, an early military submarine operated by the Confederate Army.

The CSS Hunley, as it was sometimes called, was one of the most historically significant submarines in military history because it was the first to sink an enemy warship in combat. On February 17, 1864, the Hunley attacked the USS Housatonic, ramming its spar torpedo against the enemy vessel’s hull, sending it to the ocean floor. While the attack was successful, the Hunley did not survive it, and it wasn’t seen again for over 130 years. It was found in 1995 and then recovered in 2000. The real mystery was the boat’s crew, as their remains were found sitting peacefully at their stations instead of scrambling for the exit.

It took another 17 years after the submarine was recovered for the mystery of the Hunley’s sinking and the fate of its crew to be solved. Researchers finally learned that the weapon that sank the Housatonic was fatally flawed, and the blast that destroyed its target also instantly killed the submarine’s crew.

It took over a century to locate and recover the H.L. Hunley. That’s not that unusual, as it took as long to recover a lost WWI submarine. Still, the mystery of Hunley’s demise stumped researchers for over two decades after it was found in 1995 by writer Clive Cussler and his team. Upon discovery, the eight-man crew’s corpses were located sitting serenely in their positions. This was odd, as anyone experiencing a sinking submarine wouldn’t remain seated at their position and would attempt any measure to escape and survive.

Researchers affiliated with Duke University uncovered the mystery in 2017, publishing their findings in PLOS One. They found that the Hunley was destroyed by the very weapon it employed against the USS Housatonic. The torpedo spar, which consisted of a 135-lb. black powder charge attached to a 16-foot-long pole affixed to the Hunley’s bow, delivered the killing blow. Using a 1/6th scale model, they found that the shock tube explosions and blast created a secondary blast wave that went directly back through the Hunley. According to the study, under those circumstances, each crewmember only had a 16% chance of survival. 

It should be noted that this is only one study, which hasn’t been independently verified by anyone outside of the original team. Still, there aren’t any other plausible explanations out there, and if the theory is true, it would explain why none of the submariners attempted to escape their demise, as they likely died before they had a chance to realize the truth of their situation.