A detail of a surviving relief depicting Mithras slaying a bull. This is one of those rare hidden gems well worth a visit if you're into archaeology (as I am!). This Mithraeum is concealed beneath the church of Santa Prisca, a 12th century titular church built over very ancient foundations, as is evident here, a testament to its pre-Christian past. It's located on the Aventine, dedicated to an obscure saint whose life is largely unknown. Some accounts conflate her with St. Priscilla, with whom St. Paul stayed in Corinth who eventually moved back to Rome according to a later letter. Some traditions state that the site was actually her house where both SS Peter and Paul stayed. Other accounts state that Prisca was martyred in the first century under Claudius, making her Rome's earliest martyr.
Excavations only began at this site in the 1930s, by Augustinian friars. A first-century structure was found north of the church featuring brick stamps dating to about 95 AD, so the site is certainly an ancient one. Excavators believed that it was part of an elite house, which may have even been the documented Domus Privata Triani, the house of the emperor Trajan who lived here before he began his reign, but other accounts state that the property belonged to one Lucius Licinius Sura. Notwithstanding, one of the most important features is the Mithraeum, which was added in the late second century according to a surviving graffito dated to 202. The original featured a central aisle, a niche and side benches. Not a lot is known about the rituals which took place here, but the iconography is rather consistent. Relief carvings usually show what is seen here, with some exceptions: there is a stucco Mithras the Bull Slayer, seen here in the distance. Sunbeams also feature prominently in the iconography. Mithras is usually shown killing a bull by slitting its throat with the rays of the sun illuminating the act. Mithras is depicted kneeling on the bull and here actually stabbing it, while a dog waits to lick up its blood. The god Saturn is depicted reclining, and amphorae fragments covered with stucco are also placed in the niche. The event is usually shown as taking place in a cavern, perhaps explaining the cryptic nature of the location. Renovations in 220 resulted in the expansion of the central room and the addition of smaller ones, while the frescoes were painted over with more elaborate ones, which were vital to our understanding of the cult. One depicts a procession of masked figures wearing colored tunics. Christians assumed control of the property around 400 AD and destroyed the Mithraeum, but its remains were preserved during the construction of Santa Prisca. This site is a bit difficult to access, and there aren't all that many photos of it, but if you're interested in ancient Roman religion it's definitely worth your while as it's very well preserved.