Construction workers digging at ground zero have uncovered a 40-foot pothole and other features carved by glaciers about 20,000 years ago.

Unearthing these glacial features has been critical in preparing the foundation for Tower 4 of the new World Trade Center, being built by Silverstein Properties at the southeast corner of the site. Engineers need a clear understanding of the contours of the rock.

"You want to make sure you're not perching something on a ledge," said Anthony Pontecorvo, a supervising structural engineer at Mueser Rutledge Consulting Engineers, which is working on the project.

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The bedrock could not be mapped until the soil was removed and the surface was fully exposed. But besides being an engineering necessity, the unearthing of geological features has offered scientists a rare window into the deep past.

"There are areas in local parks that have small vertical potholes exposed," Cheryl J. Moss, the senior geologist at Mueser Rutledge, told The New York Times. "But I'm not aware of anything in the city with a whole, self-contained depression on this scale."

But geology at the trade center site must take a back seat to construction. The pothole and other features are either being covered, filled in or blasted away.

"It's nice to look at," said Robert B. Reina, a supervising structural engineer at Mueser Rutledge, "but it's all got to go."