A perfect example of how layers of history overlap in Rome, this 12-century Norman church, full of beautiful Byzantine mosaics, hides much more. Down in its eerie grottos (which you explore on your own) you’ll find frescoes and mosaic floors from its previous incarnations as a 4th-century Christian church and a temple dedicated to the pagan deity Mithras—and below that, the foundations of a Roman house from the 1st century A.D., where early Christians worshipped in secret.