PHILADELPHIA For more than a century, the plot of land centered at 24th and Diamond Streets served as the final resting place for a Civil War hero, a labor leader, and about 85,000 Philadelphians.

But in 1950, city officials announced plans to move the denizens of the Odd Fellows Cemetery elsewhere in the city to make room for the Raymond Rosen housing project and a new public school.

The Rosen project is long gone, torn down in 1995 after years of crime, overcrowding, and disrepair, but the William Dick Elementary School, built in 1954, has remained - and so, apparently, have some of those interred at the Odd Fellows Cemetery, which opened in 1849.

On Wednesday, a work crew unearthed at least six pine coffins beneath the playground asphalt behind the school. Inside were human skeletal remains.

It was unclear Thursday night whose they were or how long they have been buried, said Mark McDonald, a spokesman for Mayor Nutter.

In a city as old as Philadelphia, McDonald said, it's not unusual to unearth a few bodies in the middle of a construction project.

Workers found human remains during construction of the Sister Cities Park at 18th Street and the Benjamin Franklin Parkway, and work on Kensington's Willard School, which opened in 2010, was halted when crews stumbled upon graves from a former cemetery there.

At William Dick, workers had surveyed the playground for forgotten graves before beginning work, McDonald said.

"As research was done on the parcel, we knew there was potentially something there," he said.

On Thursday, schoolchildren, neighbors, and a few school employees gathered at a chain-link fence behind the school to peer into a deep ditch where workers had been digging.

Wooden boxes, caked in dirt, were visible at regular intervals in the walls of the ditch. A smaller box, marked by orange paint, was half-buried in the middle of the ditch.

One of the children shivered.

"They say there's zombies down there," he said seriously.

Rex Purnell, 50, who stopped by the site on his bike, said his mother, Gladys, had attended the school at its opening in 1954 and remembered the old Odd Fellows Cemetery. Residents of the area had assumed the bodies had been moved, he said.

"Who are they? Who are the relatives of these people?" he said.

For the time being, work at the site - a "greening" project designed to increase green spaces in the city - has stopped, school district officials said.

A historian viewed the caskets Thursday, and an archaeologist will be brought in to assess the remains, McDonald said. He wouldn't speculate on how long the project would be delayed.

Representatives from the Odd Fellows Cemetery Co., which oversaw the cemetery, did not return a call for comment.

On Thursday, Purnell craned his neck to see through the fence and shook his head.

"This neighborhood's full of crime and shootings," he said. "But this is something totally different."