The Mithraeum underlying the Church of St. Prisca was discovered in 1934 as a result of excavation work undertaken by the Augustinian Friars. The discovery, absolutely fortuitous, was followed, in the years 1953-1966, by the archaeological investigation led by Dutch archaeologists M.J. Vermaseren and C.C. van Essen.
As often happens, the mithraeum was established in a pre-existing private home situated in the area beneath the northern part of the church and the surrounding courtyard. Equipped, on the eastern part, with a quadriporticus arranged around a garden, the house can be dated back to the end of the 1st century CE and underwent several transformations over the course of the 2nd century.
Some scholars propose to identify it as the privata Traiani, that is the house wherein the Emperor Trajan dwelled before he ascended to the throne. At the end of the 2nd century CE, a building with two aisles (perhaps the early Christian titulus) was erected over an extensive part of the complex: the present church stands upon it. More or less at the same time, the mithraeum, consisting of several underground environments covered with barrel vaults, was built.
The small entry vestibule is provided with benches along two of its walls and, in corner, with a fence, destined to the sacrifice of the victims; the vestibule gives access to the main environment, a long and narrow hall wherein used to take place the mystic repast of which the initiates would partake.
In the centre of the large niche in the wall at the end of the hall, there was placed the group representing Mithras, with his cloak billowing out, along with his dog and the bull, whereof some fragments still remain.
The two long side walls are decorated, in their upper parts, by two cycles of frescoes dating from 200 and 220 CE.