It's been centuries since archaeologists excavated Rome's central Piazza Venezia, but just a few hundred yards from the Roman Forum, skeletons of the city's past are surfacing.

In a hole 18 feet below the piazza, construction workers and archaeologists digging Rome's new subway line are carefully uncovering Rome's layered history.

Closest to the surface are building remnants from renaissance Rome that were torn down in the late 19th century. The next layer exposes the remains of Via Flamina, a medieval road that once cut through the city. Below that, herringbone pavement from around A.D. 700 peeks through.

Unlike Paris or New York, whose intricate subway lines connect, Rome has historic subterreanean layers that have prevented its subway system from uniting. Giovanni Simonacci, the technical director of the new subway line, says that in Rome, archaeological remnants can be found up to 30 feet below the surface. At 90 feet down, the subway's tunnels won't displace antique artifacts, but the upper stations and air vents will.

Archaeologists decide whether artifacts are historically important enough to be maintained, or whether they can be destroyed. They deemed a Roman tavern from the Middle Ages acceptable for destruction, but they scrapped an entire subway stop near the Pantheon after workers found the base of an imperial Roman public building. Tourists will walk an extra 200 yards from Piazza Venezia to reach the relocated stop.

Elsewhere at the excavation, workers have found what archaeologists call a "looter's hole." During the Middle Ages, builders tied to a rope would drop down the well-like shaft and roam the lower layers in search of bricks, blocks of rock or slabs of marble from earlier centuries to be used for new construction.

Simonacci says the Piazza Venezia dig shows just a fragment of the rich Roman heritage below ground.

"There isn't an inch of Rome that doesn't have some artifacts below the street," he says. "In 300 A.D., Rome already had one-and-a-half million inhabitants. If we were to bring to light everything they and subsequent generations built, we would have to eliminate all of the streets of Rome."