In 1995, researchers discovered the wreck of the Confederate Navy’s submarine, the H.L. Hunley, the first combat submarine in history to sink another ship. In 2000, they were able to raise the sub, including the remains of the eight sailors aboard. But the Hunley presented a mystery—soon after jamming a rudimentary torpedo into the side of the U.S.S. Housatonic on Feb. 17, 1864, the submarine also sank, though there are no signs that it was attacked or damaged. Now, reports Brandon Specktor at LiveScience, a new finding from the sub may give some clues.

Since being raised, archaeologists and conservators have painstakingly excavated 1,200 pounds of concretion—rock-hard silt and sand that accumulated on the 40-foot-long, sausage-shaped craft as it sat four miles off the coast of Charleston, South Carolina, for 150 years. Researchers recently worked on removing the concretion from and conserving eight cast-iron keel blocks, weighing about half a ton total, that had been used to steady the submarine. But they also found the largest blocks were connected to a quick-release mechanism, meaning if there was any trouble the crew could eject the blocks and quickly rise to the surface. Recent work on the sub shows that the three levers of the release mechanism, however, were never engaged and the keel blocks are all in place, meaning the crew never tried to use the safety feature. That suggests that they either did not think they were in trouble or were incapacitated before the boat went down.

“As a diver, your first instinct if you’re in trouble is to get to the surface by releasing your weight belt, and it’s part of your training,” Johanna Rivera, a conservator on the project, tells Bo Peterson at The Post and Courier. “The keel blocks serve the same purpose, so it appears there was no sense of panic (among the crew)…[The finding] is an extra layer of complexity as to what really happened.”

We may never know exactly what happened, but the keel block narrows things down. Jeffrey Collins at the AP reports that one theory is that the submarine got stuck in the mud while waiting for the tide to turn so it could make it back to dock after it jammed its load of powder into the Housatonic. If that was the case, however, the crew might have been able to drop the keel blocks to get themselves off the bottom, which did not happen. Another theory is that, soon after the explosion on the Union ship, another ship rushing to its aid struck the Hunley, incapacitating the submarine and leading to its doom.

Perhaps the most compelling idea is that the blast from the exploding Housatanic was enough to knock out the crew. In 2017, researchers released a paper arguing just that. Charles Q. Choi at LiveScience reports that the Hunley jammed 135 pounds of black powder into the hull of the enemy ship below the water line. The resulting blast wave, the study shows, was enough to knock the crew out and rupture their lungs. “Blast injuries are consistent with the way the remains were found inside the boat, as blast waves would not have left marks on the skeletons, and would not have provided the crew with the chance to try to escape,” lead author Rachel Lance, a biomechanist at Duke University, tells Choi. “Blast waves are capable of inflicting lethal injuries on someone without ever physically moving them.”

The keel blocks, along with the fact that the crew never used the bilge pump, suggest the crew did not take evasive action as the submarine sank. While the clues mount up, archaeologist Michael Scafuri, who has worked on the project for 18 years, says we keep learning more about the sub, but may never know exactly why it sank.“I would love to get to that point absolutely…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,” he tells the AP. “After all, we don't have the blueprints.”

In fact, the Hunley sank three times during its brief career, which lasted from July 1863 to February 1864. The first time it sank at dock, killing five crew members aboard. In October 1863, it sank during a demonstration, taking with it a crew of eight, including its inventor Horace Hunley. In that case, the crew had tried to drop the keel weights, but they were too late.

In 2004, the crew aboard the Hunley during the third and final sinking were buried in Charleston near the other crews that had also perished in the sub.

The newly conserved keel weights will go on display at the Clemson University’s Warren Lasch Conservation Center where visitors can see the Hunley and artifacts recovered from it during weekend tours.