They include entire historical city centres, such as Rome, Florence and Venice, and archaeological areas such as the ruins of Pompeii and Herculaneum.
Work to locate the starting point of the Appian Way, believed to be some eight metres under the ground, has so far been complicated by groundwater.
Nevertheless, digging in higher strata of ground has unearthed relics from different periods, including a marble bust from the second century A.D. and an early papal square coin, minted between 690 and 730.
So far, the excavation has reached residential or commercial structures dating from the time of Emperor Hadrian, who died in 138 A.D.