The Rome Index
Basilica of San Clemente
Three layers of Rome in one building — medieval church, 4th-century basilica, and a Mithraic temple below.
What it is
A working 12th-century basilica built over a 4th-century church, which was itself built over a 1st-century Roman building containing a Mithraic temple — one of the clearest illustrations of how Rome's layers sit directly on top of each other.
Going underground
A ticket covers the descent through the lower levels, where you can hear — and sometimes see — an underground stream that still runs through the lowest Mithraic level.
The mosaics
The 12th-century apse mosaic in the upper basilica is one of the finest in Rome, depicting the Triumph of the Cross with remarkable detail and color.
The archaeological funding story
Excavation of the lower levels was carried out by Irish Dominican friars starting in the 1850s, a project that took decades and remains an active point of institutional pride for the order that still administers the church today — a detail that adds a layer of modern history to the ancient one.
The sound of running water
One of the most memorable details of the visit is genuinely audible running water at the lowest level — an ancient underground stream still flows through the site, channeled but not eliminated by the various structures built above it over two millennia.
