The Basilica of St Clement (Basilica di San Clemente al Laterano) is an impressive 11-12th century church in the centre of Rome. Excavations beneath it reveal multiple layers of earlier worship and occupation.
The first level underground holds the remains of a 4th century church, itself converted from a high class dwelling. This church contains some splendid frescoes dating from around the 8th century. Yet further below ground can be seen that the basement of the dwelling originally held a 2nd century mithraeum.
Archaeological excavations reveal that even before these structures, a fine villa and warehouse (possible mint?) were on the site. Both of these structures were destroyed by fire in AD 64. At the deepest level can be heard the flow of an underground stream, presumably culverted in the early Roman era.
All of the underground features can be [visited] (https://www.basilicasanclemente.com/eng/booking/) for a self-guided tour of an intriguing site.