OSSINING, N.Y. It started with an ever-expanding sinkhole at the entrance of the Mystic Pointe condominiums here and led to an excavation this spring that revealed an underground complex of brick chambers with vaulted ceilings.

Now the subterranean structure, believed to date to the mid-19th century, is a mystery just begging to be solved. Is it as pedestrian as a root cellar? Or as storied as a stop on the Underground Railroad? Does it stretch beyond the cluster of at least nine known rooms to connect to tunnels elsewhere?

An ad-hoc group of residents, local historians and archaeologists in this Westchester County suburb is racing to figure it out before road repairs that could lead to the destruction of the rooms, which sit under a wooded area that had been part of a Victorian estate and once was owned by a Catholic church.