Archaeologists surveying the site of a new hotel in Alexandria, Virginia have uncovered the remains of a Revolutionary War-era ship buried in the mud of the Potomac River. After centuries of its muddy entombment, what’s left of the ship could give researchers new hints as to how 18th century colonists in North America built their boats, Patricia Sullivan reports for the Washington Post.

This isn’t the first time crews working at this particular construction site have dug up historical artifacts: just a few months ago, archaeologists along with construction workers uncovered the remains of an 18th century warehouse they believe was Alexandria’s first public building. But unlike that find, which was marked on historical maps of the city’s waterfront, there was no record of this particular ship.

“It’s very rare. This almost never happens,” archaeologist Dan Baicy tells Sullivan. He works for Thunderbird Archaeology, the firm in charge of excavating the construction site. “In 15 years that I’ve done this work, I’ve never run into this kind of preservation in an urban environment where there’s so much disturbance.”

Researchers have long known that construction along Alexandria’s waterfront could uncover the remains of sunken ships, as late-18th century workers once used them as the framework for the landfill process used to extend the waterfront, Mary Ann Barton writes for Old Town Alexandria Patch. But finding two well-preserved historical finds at the same construction site is a different story.

“This is like the jewel in the crown for us right now,” Thunderbird’s principal archaeologist, John Mullen, tells Sullivan.

Construction crews were first alerted to the ship’s existence in December 2015 when they uncovered it’s bow during the site’s excavation, Sullivan reports. Since then, archaeologists dug up almost a third of the original hull, stretching nearly 50 feet from keel, to framework, to stern. It’s unclear exactly what the ship was originally used for or who built it, but researchers suspect that it could have been a large cargo ship or a military vessel.

According to Baicy, the ship’s wooden hull was well preserved thanks to the oxygen-starved mud it was buried in. Without air bubbles to help speed along the ship’s decomposition, the remains survived in the murky, muddy silt of the Potomac. That’s not the only reason the find is so special: Old Town Alexandria’s waterfront has been a popular place for construction for centuries, and Baicy says that workers laying down a brick footing for a later warehouse “barely missed the boat,” Sullivan writes.

This week, archaeologists will document the ship with 3D scans, photographs, and drawings on site before removing its remains for storage in tanks of water until it can be studied by a preservation laboratory.

Meanwhile, archaeologists will continue to survey the site, where Baicy’s team has also uncovered three separate outhouses that back in the day doubled as colonial trash cans—a veritable treasure trove for archaeologists wanting to learn more about how the early American colonists lived.

Excavation of these sites has already begun. What were their early findings? “For some reason, we’ve found a lot of shoes,” Baicy tells Sullivan.