This is a fascinating find that is not on the major tourist sight list. There is a 12th century church named after Clement, a 1st century church leader in Rome (who is acclaimed as the third pope), which is pleasant, but it is part of the latest layer of the history of the site which is now administered by Irish Dominicans. It was the Irish Dominicans who discovered that there was an earlier church on the site and started excavations, and discovered not only a 4th century church, but a deeper earlier layer also.

The church is lovely with 6th century marble choir section that was originally in the earlier church and then moved here and simply looks elegant. Also there is an incredible golden mosaic in the apse based around the theme of the Cross as tree of life which is simply beautiful. It's considered that this mosaic was somewhat of a copy of an apse mosaic in the 4th century church below, as it seems to reflects a style and content more suited to the fourth century than the thirteenth.

Underneath the church are a further two layers of history that have been partially excavated. The bottom layer is remains of Roman houses and a Mithraic temple which were built after the fire of 64AD that had destroyed the area. You can also hear and see an underground river rushing by, making its way to the Tiber, which gives the excavations a sense of unease that the foundations are built on water.

Above this is the middle layer of a church that dated back to 392AD. Some parts of the church has yet to be excavated and other parts remain bricked up. The structure that has been unearthed had a nave with an aisle on either side but there have been walls built between the pillars. There are several paintings on the walls depicting bible scenes and scenes connected to saints, but they are quite faded and others are fragmented. They are quite interesting if you are interesting in early church history and its impact on artwork, something I have done some study in, but they are probably not that fascinating to most other people.

Entrance to the excavations costs 5 euros but note that they are very warm and sticky and that no photography is permitted. There is a small courtyard outside the church which is pleasant to refresh yourself after the breathless underground experience. Although it is possible to see other early churches in Rome, I loved San Clemente as it provided at least three layers of history and showed how Rome had changed and developed over time from a pagan city to being a great centre of Christianity. It is worth a visit to get a sense that Rome is the eternal city that is constantly evolving.

Paying a small fee will gain you entrance into that lower church, which dates to the 4th century. The original church is a little difficult to recreate mentally, as much of the basilica, such as the spaces in between columns, has been filled in for support. However, ancient frescoes and altars remain, giving a glimpse into one of the first churches built in Rome. (If you're wondering why the church was built over, it was largely destroyed by the Normans, and the decision was made to simply fill in the old one and build on top of it.)

Walk a little farther through the musty underground corridors and you'll find yourself in an ancient Roman home. Because of the placement of the church, it is believed that this home once served as a secret place of worship for Christians, back before Christianity was legalized. The home is also believed to have been partially destroyed during Nero's infamous fire (which seems likely, given its close proximity to the Colosseum).

Continue on and you can peek into a one-room pagan shrine. As a remnant of the testosterone-driven Cult of Mithras, the room successfully brings the building full circle from its ancient pagan roots to its modern Christian function.

Add in a glimpse of the ancient Roman sewer system (that's the almost omnipresent rushing water that you're hearing) and a few creepy staircases leading to now-empty crypts and San Clemente provides an often-overlooked vision of Roman history. Some of the more ancient spots can be difficult to interpret, but it’s definitely worth a visit if you have the time and a good guidebook.

This is one of the oldest churches in Rome, dating from the fourth century. We entered the top, which dates from medieval times, and is dominated by beautiful mosaics with tons of gold. The mosaics were so beautiful that I got a book just on the mosaics, knowing that my photographs would not do them justice. There was also in the upper church, which is an ancient basilica in structure (big rectangle with columns separating out side sections) a chapel to Saint Catherine of Alexandria with some beautiful early Renaissance frescoes, including ones with Catherine (of Alexandria) with her wheel.

We descended into the rough church below, which was used between the fourth and eleventh centuries. There were some wall paintings there, also, particularly of Clement and some other saints.

We descended even further to an old Roman village and the temple and school (seminary) dedicated to Mithras. The highlights of this were an altar to Mithras that is in very good shape.