The tombs, which city archaeologists knew existed but were unsure exactly where, were discovered during a water main dig and are about two centuries old

City workers have discovered two burial vaults underneath Washington Square Park in New York City, uncovering the remains of at least a dozen people interred around two centuries ago.

Contractors for the city department of design and construction (DDC) uncovered the first vault on Tuesday, during excavations to replace a century-old water main on the east side of the park, in the heart of bustling Greenwich Village. The workers called an archaeologist contracted by the city, who opened a way into the chamber only 3.5ft beneath the sidewalk.

Inside they found an arched brick chamber with skulls, femurs and other bones littered on the dirt floor.

The first vault was actually a rediscovery: power company ConEdison first uncovered the vault in 1965, finding 25 skeletons inside. Before this week’s excavation archaeologists knew the tomb existed, but were not sure where thanks to the company’s poor record-keeping.

Disarticulated skeletons in the first vault. Photograph: DDC

“It’s the second vault we didn’t expect,” Alyssa Loorya, an archaeologist contracted by the DDC told the Guardian. Late Thursday she and her colleagues with Chrysalis Archaeology, a company that works with the city on such projects, found an identical chamber slightly south of the vault.

In contrast to the first, disturbed chamber, the second vault contains about 20 wooden coffins, at least some with name and date plates bolted on their sides. They are built identically: 15ft wide, 27ft long and an estimated 8ft from ceiling to floor. In the second vault a wooden door with diagonal slats guards the room, hanging by iron or copper hinges, its lock apparently intact.

DDC commissioner Feniosky Peña-Mora said the archaeologists are proceeding with caution – “we don’t want to do any more disturbing than we need to” – and have no plans yet to enter the chamber or move the bodies. City policy at archaeological sites is to leave remains in place, whenever possible, out of respect for the dead.

But Peña-Mora noted tantalizingly that it’s not clear where the vault doors, facing westward under the park, even lead.

Facebook Twitter Pinterest The excavation at Washington Square Park. Photograph: The Guardian

For now researchers will continue to work from a distance with high-resolution cameras, telescope lenses and possibly a boom stick to get new angles on the remains and coffins.

“Of the images we’ve processed so far, we’re able to make out the beginning of a year of 18,” Loorya said. “Of course no further than that – yet.”

She added that with photos of a high enough resolution, archaeologists could glean a range of biological information. “You can get enough resolution to see dental wear patterns, suture closings on bones,” she said, in addition to “indicators of age and sex” and signs of disease.

They have so far counted about 12 skulls among the disarticulated skeletons of the first chamber, but have yet to process all the images from both vaults.

Now a leafy centerpiece of lower Manhattan, surrounded by university buildings and bars and recognized for its huge arch and fountain, the plain where Washington Square Park stands was a graveyard in the early 19th century. Not long after the revolutionary war the area served as a potter’s field for the poor and criminals of New York, most buried without ceremony or headstone.

Loorya said her team is for now dating the vaults during this period of the early 19th century, noting that before the cemetery was leveled in 1826 and declared a parade ground, churches bought plots adjacent to the field.

Only a few years earlier the site also served as a final resting place for victims of a yellow fever epidemic. In the early 1800s city lawmakers decreed that no church could bury its dead below Grand Street, pushing churches to claim plots near the field. There is no definitive documentation of how many people are buried under the park and neighborhood, Loorya said.

Human remains and gravestones periodically turn up around New York, and some estimate there are thousands of dead beneath the city. Bryant Park was similarly used as a potter’s field in the 19th century, though the remains there were eventually moved.

The most likely church to own the Washington Square plot, Loorya said, was the Cedar Street Presbyterian church, a congregation that splintered off from the Scotch Presbyterians in the late 18th century. But the church has no official records, she said, leaving the researchers to hope that a pastor may have kept personal records when he moved upstate.

“We’ve actually started making a timeline of events, and like a flow chart to try and track it,” she said.

“Like CSI,” Peña-Mora joked. He added more solemnly: “It’s our responsibility to makes sure this doesn’t happen again 100 years, or 50 years from now.”

Coffins in the second vault. Photograph: DDC

The DDC has closed off the site to vehicles and pedestrians and is redesigning its project to accommodate the dig, department spokesperson Shavone Williams said. The department and researchers know the northern boundary of the cemetery but not its southern one, meaning there is an outside chance of more tombs under the street.

Loorya said the site is significant but added that artifacts and burials are scattered under the city often in well-preserved form, especially in the southern, older neighborhoods of Manhattan. South Street Seaport, for instance, has for years been treated as an archaeological site, and in the ruins of the World Trade Center workers found the remains of a 17th century ship.

“Despite massive amounts of disturbance from utilities, even subway installations, we still find pockets of either disturbed or undisturbed materials,” Loorya said.