Wilmington, North Carolina, was the Confederacy’s last port. During the Civil War, blockade runners that evaded the Union navy were one of the Confederate army’s few lifelines. But maneuvering a small, fast ship in the dead of night was hard as it sounds, and many of them didn’t make it. Archaeologists now think they’ve found the shipwreck of one of those long lost blockade runners off Wilmington: the Agnes E. Fry.

At least they’re “99 percent sure” it’s the Agnes E. Fry, says archaeologist Billy Ray Morris—this being an extremely old boat under extremely dark waters. The last one percent of uncertainty will be worked out next week, when divers descend to the shipwreck with a sonar machine to map the shipwreck in glorious 3D detail.

The search for Agnes E. Fry is part of a larger effort to map the underwater artifacts of Civil War battles. When Morris, deputy state archaeologist for North Carolina, and Gordon Watts, director of the Institute for International Maritime Research, recently resurveyed the waters around North Carolina, they found previously mapped shipwrecks were more exposed than ever—likely due to dredging in the river that feeds into the Atlantic near Wilmington. Three blockade runners had wrecked off the coast near Oak Island but never been found. What if they too were newly uncovered?

So the archaeologists ran their survey boat around Oak Island. The boat carried a magnetometer—think a metal detector that can detect distortions in the Earth’s magnetic field caused by a ship’s iron hull. And then it had a side scan sonar device, which trailed behind the boat sending sound waves and measuring their bounce off the ocean floor. Lo and behold, they found something.

North Carolina Department of Natural and Cultural Resources

Morris had done archival research in Bermuda—where many of the blockade runners stocked up with supplies before heading to North Carolina. From that, he knew that among the three missing boats, only the Agnes E. Fry was longer than 200 feet. This shipwreck was 225 feet long. Salvage records also showed the ship should be missing its paddlewheel and engine. (Blockade runners were optimized for stealth: Their topline engines only burned smokeless high grade coal, and their paddlewheel blades were designed to prevent splashing. These were valuable parts.) Indeed, the ship was missing its engine and paddlewheel. Otherwise, it was remarkably well preserved.

Divers also went down to check the wreck out with their own, erh, hands. Eyes are useless here because the water is black with tannins from trees upriver. “Imagine sticking your face in a cup of black coffee and opening your eyes,” says Morris, who has been diving off the coast of North Carolina since he was a kid. In other words, he’s used to being blind underwater. By groping around in the dark and knowing the length of his wingspan and kick cycle, he can get a pretty good idea of what the shipwreck looks like.

But with this upcoming sonar scan, which uses a different type of sonar called sector scan sonar, everyone will be able to see what the shipwreck looks like—not just in 2-D but in 3-D. Once initial word of the shipwreck got out, Morris began getting calls from people who wanted to help, including the great-great-granddaughter of the Agnes E. Fry’s chief engineer who had his journal describing the night his ship ran aground.

He also heard from the Charlotte Fire Department and their special operations divers, who wanted to bring in a sector scan sonar machine from Nautilus Marine Group International to check out the shipwreck. “In really poor water conditions, those are your only options,” says Matthew Johnson-Roberson, an engineer at the University of Michigan who has also worked with Nautilus for underwater archaeology. “The problem is they’re expensive.” But that won’t be a problem here: The Charlotte Fire Department and Nautilus are volunteering their services.

The divers will bring the sector scan sonar device down to the ocean bottom and set it up on a tripod to send out sound waves. The ones that bounce back will get converted into a visual snapshot of the ocean floor. “All of this is made light years easier by high speed computers,” says Morris. While Morris and others are diving, someone on the dive boat with be checking out the data in real time on a laptop.

What Morris is most curious about is whether the cargo holds of the ship are still full. From the engineer’s journal, he knows that the ship’s crew probably didn’t finishing unloading their grounded ship before they fled. And the commander of Wilmington’s Fort Fisher had written a letter asking specifically about the whereabouts of the Agnes E. Fry. Why was the commander so interested? Morris might finally solve the mystery.