It is a commonplace to point out that Rome's rapid rise to power in the ancient world resulted in great measure from its position in the Italian peninsula. Surrounded by mountains on all sides except on the south the Urbs is situated in the midst of a plain of volcanic origin, known as the Campagna. The Tiber flows through the city from north to south on its way to Ostia, the ancient seaport, where it empties into the ocean. Although the seven traditional hills upon which the city was built still retain their ancient names and may still be traced, in some cases their elevation is so slight as to be disappointing to those who visit Rome for the first time. What then has caused the changes in ground level which have taken place since the ancient period? It seems clear that many of the elaborate buildings of the Empire were superimposed upon the foundations of earlier structures which had not been completely demolished. Then too, as parts of ancient Rome fell into ruin during the Middle Ages, the débris was not cleared away, but structures of the Renaissance period were build upon the accumulated rubbish. Views of the Roman Forum dating from the sixteenth and seventeenth centuries show the columns of the ancient temples buried deep in the earth, a clear indication of the extent to which the ground level had risen. In seeking to ferret out the secrets buried beneath the . .