It was the first submarine in history to successfully sink an enemy ship. Made out of 40 feet of bulletproof iron, the H.L. Hunley was a Confederate submarine with a crew of eight. But despite its claim to fame, it was a dangerous vessel to be inside.

In a career of just eight months during the Civil War, between July 1863 and February 1864, the sub sank three times, killing nearly 30 men—including its inventor. (It was recovered twice.) Its final sinking, shortly after it plunged a live torpedo into the hull of the Union warship USS Housatonic, has long mystified maritime and military historians. What sank the Hunley for good?

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Over 130 years after it sank, the submarine was found on the seabed in 1995. Five years later, it was brought to the surface. Inside, all eight crew members were eerily in position at their stations, around a giant hand-crank that ran down the middle of the sub. The discovery has generated a raft of possible theories as to why it sank, and why the crew aboard didn’t seem to make any attempt to escape.

Researchers have found another piece to the puzzle: A hidden failsafe mechanism in the Hunley’s keel should have helped the crew escape the vessel, but it was never activated. This suggests the crew may not have seen whatever sunk the sub coming.

Archaeologist Michael Scafuri has been working on the submarine for 18 years. After removing layers of corrosion, silt and shells from the sub, his team of researchers found that the emergency levers were all locked in position. “It’s more evidence there wasn’t much of a panic on board,” Scafuri told the Associated Press. The levers would have released 1,000 pounds of so-called “keel blocks,” bringing the submarine up to the surface and allowing the crew to swim away to safety.

The discovery suggests two options: The crew may not have realized they were in danger, or not anticipated a need to surface quickly. The Hunley was small and cramped—not even large enough for its crew to stand up straight—yet its men showed no attempt even to get away from where they were stationed.

It’s why researchers at Duke University proposed that they must have been killed instantly, perhaps by the blast from the submarine’s own spar torpedo. “The pressure wave from the explosion was transmitted into the submarine,” graduate student Rachel Lance told Nature. “It was sufficiently large that the crew were killed.” In their 2017 study, researchers made a scale model of the submarine, then blew it up in a pond. By measuring the forces, they finally had the data to back up a long-held suspicion.

Despite these advancements, Scafuri says they were still a while away from being able to say definitively what had happened inside the submarine. “I would love to get to that point absolutely,” he said, but made no promises about whether it would be possible.

For now, scientists are focusing on the excavation, removing more of the corrosion and underwater matter from decades on the ocean floor. Each step tells them more about the craft and its crew, from what their faces were like to the lucky gold token found in the captain’s pocket.

What’s more, the scientists are finally beginning to get a handle on the inner workings of this thoroughly analog piece of military technology. “We keep seeing parts that no one has seen in 150 years. All of them add into the mix of what happened and how this sub was operated,” Scafuri said. “After all, we don’t have the blueprints.”

READ MORE: The Hunley's Daring Submarine Mission