Crews excavating the site of the World Trade Centre for the foundations of a skyscraper have uncovered features carved into the bedrock by glaciers about 20,000 years ago, including a 12-metre deep pothole. Exposing the rock was critical, said Anthony Pontecorvo, a structural engineer. "You want to make sure you're not perching something on a ledge." Cheryl Moss, senior geologist with the consulting engineers, said: "There are areas in local parks that have small vertical potholes exposed, but I'm not aware of anything in the city with a whole, self-contained depression on this scale."