A Cave Built Indoors

Every mithraeum is the same room. It is small, it holds a few dozen people at most, it has no windows, and it is arranged as a long vaulted space with a raised bench down each side and the image of Mithras killing the bull at the far end. The vault is a cave, and where no natural cave was available one was constructed, which is what happened under the Aventine.

Manfred Clauss notes that the consistency of the plan across the empire, from the Rhine forts to Rome to Syria, is one of the strongest pieces of evidence that this was a single organised cult and not a loose family of local practices. The benches tell you what the room was for. Members reclined along them and ate, facing each other across a narrow floor, with the bull slaying scene in view at the end. The size caps the congregation, and that is a structural fact about the religion: a mithraeum could not become a mass movement in the shape it had, because when a group outgrew the room the answer was another room rather than a bigger one. Rome held a large number of them. The Santa Prisca sanctuary, excavated from 1953 beneath a church on the Aventine hill, is among the best preserved anywhere in the empire, and its particular interest is that its walls were painted and a large part of the paint survived the centuries of burial that followed.