It’s early morning on the coast of Charleston. Archaeologist James Spirek and his team of researchers are gearing up to solve a maritime mystery. A centuries-old cold case lies beneath the water’s surface: What happened during the battle between the H.L. Hunley and the U.S.S. Housatonic?

“We’re trying to work through all these events, trying to figure out what is the evidence of this explosion,” said Spirek, an archaeologist with the South Carolina Institute of Archaeology and Anthropology.

During the civil war, a Union blockade on the port of Charleston led to fierce fighting on the water. A Confederate submarine, the H.L. Hunley, torpedoed and struck down a Union battleship, the U.S.S. Housatonic, on Feb. 17, 1864. The Hunley made history, becoming the first submarine to successfully sink an enemy warship. But just like the Housatonic, the Hunley never made it back to port that night. Instead, the submarine disappeared, leaving a gap in history researchers are trying to fill.

“This is a landmark event in naval history,” Michael Scafuri, senior archaeologist at the Warren Lasch Conservation Center in Charleston, said.

The Hunley was found and recovered in the late 1990s and is currently housed at the conservation center. Still, researchers have run into a roadblock after examining the Hunley’s wreckage. They still don’t know why the submarine never made it back to port.

“Think about it as a scene from crime scene investigators of a really cold case,” Scafuri said. “A lot of the evidence fades away.”

The lack of evidence is now forcing researchers to return back to the Housatonic wreckage site, hoping they’ll find more clues.

“The only new place we’re going to get physical evidence is going to be the wreck of the Housatonic where the actual attack took place,” Scafuri said.

That’s where Spirek and his team come in. For the past month, the team has set out every day to the wreckage site, which sits about 5 miles out to sea on the ocean floor. After arriving at the wreckage site, the crew suits up and dives 30 feet to the ocean floor. Battling murky water, the team uses ropes and hand signals to guide them to the ship.

Using a process called hydro probing, divers tap the wreckage with 10-foot poles, hoping the sounds will lead them to the ship’s hull.

“That’s where we believe most of the damage would’ve been preserved,” Scafuri said.

The divers travel meter by meter, using what looks to be an adult version of an Etch a Sketch-style device to chart where each sound happened and at what distance along the wreckage.

The crew essentially is trying to solve a puzzle in the dark. With most of the wreckage buried beneath sediment and sand, the crew has to rely on their ears rather than their eyes.

And even though Spirek and his team have yet to crack the case, this mapping expedition will help future researchers get a better idea of the wreckage area on the ocean floor.

For now, the mystery of what happened to the H.L. Hunley remains.

“As of now, there’s still no clear indication of why the crew passed away after they detonated their torpedo,” Scafuri said.

The crew will finish its expedition at the end of October. Spirek’s team hopes to return to the Housatonic wreckage site for more clues sometime next year.