The 12th-century Basilica and the ancient site of San Clemente, two of Rome's hidden gems, are just a short distance from the Colosseum. Gasp in awe as you explore this location's three underground floors to learn about its more than 2,000-year history. The underground floors of the Basilica were undiscovered until 1860. If it wasn't for a flood in that year that prompted an Irish Dominican Roman Catholic priest and archaeologist to discover this space, it would have been undiscovered even today.

You can go 100 feet underground even if the excavations aren't finished yet. On the lower level is a basilica from the fourth century. When the 12th-century basilica above it was built, it had long been forgotten. Further down, there is an antique Mithraic temple with an altar.