Workers upgrading water mains under New York City’s Washington Square Park this week discovered a vault containing a large pile of skeletal remains dating back approximately 200 years.

Officials from the city’s Department of Design and Construction told Newsday that the vault measured 8 feet deep, 15 feet wide and 20 feet long. It contained the remains of at least a dozen people. Anthropologists and archaeologists would be asked to investigate the vault to determine its exact age.

The agency told DNAInfo that work on the project would continue as planned south of the burial site. The vault was discovered east of the park, in an area surrounded by New York University buildings.

The land where the park is now located was a so-called potter’s field, or public burial ground, between 1797 and 1825. The city bought the land the following year and turned it into a military parade ground, and then a park.

Historians have estimated that approximately 20,000 people were buried underneath the park.