The place is the Basilica of San Clemente. It's not far from the Colosseum, a short way up the Coelian Hill. It's a church operated by the Irish Dominicans ever since the Pope gave it to them in the 1600s as a refuge from the persecutions of Cromwell. Like many medieval churches in Rome, you can just walk in during the day and look at the architecture and art. The current active structure was built in the 12th century. There's a guy asking for money near the door, but I wasn't quite sure whether he was actually affiliated with the church, so I didn't put any money on his plate.
Once you're inside the church, there's a small ticket office where you can pay 10 euros to see the "archaeological area" as they (under)state it. For God's sake, do it. It was the best 10 euros I spent on the whole trip.
What is the "archaeological area," you ask? In 1857, the Prior decided to excavate under the basilica. Why? Maybe he was just a bored Irishman making his own fun, as the Irish will do. One imagines there were rumors of ancient chambers under the structure, but there are probably rumors like that about every building in Rome. Perhaps he was talked into it by de Rossi, an archaeologist who supervised the excavations with him.
From some maps I bought in the bookshop, it looks like they initially dug into the floor of the sacristy along an interior wall. It turned out that the wall was the outer wall of an entire earlier basilica (the sacristy protrudes from the floor plan of the earlier basilica.) That basilica, which was actually somewhat larger than the current structure, was built in the 4th century. After several decades of excavation, they had nearly completely uncovered the 4th-century basilica. They observed that columns supporting the roof of the old basilica had been incorporated into foundation walls, the rooms had been filled with rubble, and the walls of the new basilica had been built more or less directly on top of the lower walls.
Unfortunately the floor of the new basilica was built slightly lower than the ceiling of the old one, such that a lot of late-antiquity mosaics and frescoes were cut off about 3/4 of the way up. I am no judge of medieval religious art, but they are said to be quite significant. It is suggested that one fresco is a portrait from life of Theodora (500-548), the controversial wife of the Emperor Justinian. The bottom of one badly deteriorated fresco reads, "Whosoever may read these letters of my name, let him say: God have mercy on unworthy John." I read those letters and said what he asked!
Oh, by the way, it is believed that St. Cyril was buried in a corner of the 4th-century basilica. You know, St. Cyril, who with his brother Methodius brought Christianity to the Slavs, including a translation of the Bible using the first Slavonic alphabet, which they invented and which evolved into Cyrillic? There were a couple of Russians there to see Cyril's original burial place. The remains were lost in 1798 but supposedly some were returned in 1963.
There's also a fresco containing what is claimed to be the oldest existing written Italian, as opposed to Classical Latin. The sentences contain the phrase, "sons of bitches."
My favorite thing was a marble slab with a pagan inscription on one side that was later flipped over and re-used with a Christian inscription on the other side. They have the thing mounted on an axle so you can turn it over and see each side. That's almost too perfect - I wonder if it is some kind of fake.
After digging out the 4th-century basilica, the archaeologists realized that it was actually the converted second story of a commercial building dating from the 1st century. So they dug out a portion of the ground floor of that building, a narrow alleyway, and another building across the alleyway. The other building was a private home that contained a Mithraeum --- an altar of the Mithraic religion, which was briefly a competitor to Christianity in the waning days of the Roman religion.
Apparently the Mithras cult always worshipped in secret, underground, so the Mithraeum was probably in use at the same time as the 4th-century basilica, after the 1st-century street level had been raised one story. It would have been situated as the basement of the building across the street from the basilica. The Mithraeum would have been abandoned in the late 4th century when the emperor Theodosius banned all pagan religions.
In the very bottom corner of the 1st-century structure, a large natural spring issues forth and drains through a duct built in 1937 that joins with a larger discharge, the preclassical Cloaca Maxima, that runs under the Forum to the Tiber River. Before that duct was built, the lower level was full of water and unreachable.
I only relate here the things that made a big impression on me. There is a lot more to it, and in fact, a lot more that could be excavated if they had the funds. You can learn more and donate at the basilica's website.
San Clemente is not exactly obscure (I learned about it from a TV show) but I bet not one in a thousand visitors to Rome goes there. At times in the 1st-century level, I could not see or hear another person, and it's dark and damp. I'd call it creepy, but a better word perhaps is ominous. I could feel the presence of others. This place made an enormous impression on me. It is the kind of thing we simply cannot experience anywhere in the New World.