Basilica of San Clemente

What looks like a fairly standard medieval basilica at street level opens, via a staircase inside, onto an excavated earlier church below, and then further down onto Roman-era rooms including an ancient temple to Mithras with its altar still in place. This vertical excavation, with underground water still audibly flowing at the lowest level, makes it one of the clearest illustrations anywhere of how modern Rome is literally built on its own past.

Ground-level entry is free, while access to the excavated lower levels requires a ticket. It's a favorite among visitors who want real depth, in every sense, rather than another single-era church.