In living cities, archaeologists typically get to muck around underground during the construction of parking lots, with digs up to 26 feet below ground. With its subway lines traveling at nearly 100 feet below ground, Line C has given archaeologists access to artifacts dating as far back as the Paleolithic era.

“We haven’t done anything so extensive or gone so deeply” for years, Ms. Rea said.

As tens of thousands of cubic meters of earth has been moved during the line’s decade-long construction, each unearthed artifact — marble capitals and mosaics, and even remains of long-ago leftover meals and the ruins of 19th-century villas — has been painstakingly documented, cataloged and extracted. Some will go on show once a proper exhibition space is found. Some more monumental finds will be recomposed to be admired in situ.

“We’ve found enough materials to fill a warehouse” in a Roman suburb where they are being studied, said Francesco Prosperetti, a special superintendent with archaeological oversight.

The excavations also turned up ancient structures.

A huge public building known as the Auditoria, dating to Hadrian’s time in the second century, came to light under the central Piazza Venezia during an exploratory phase. Archaeologists believe the two-story building was Rome’s first university, used for cultural events and lessons. Today the square is a busy crossroad for traffic and tourists.