The Mithraeum was either an adapted natural cave, cavern, or building imitating a cave. Where possible, the Mithraeum was constructed within or below an existing building, such as the Mithraeum found beneath the Basilica of San Clemente in Rome. While most Mithraea are underground, some feature openings in the ceiling to allow light to enter, a reminder of the connection to the universe and the passage of time. The site of a Mithraeum may also be identified by its singular entrance or vestibule, which stands across from an apse at the back of which stands an altar on a pedestal, often in a recess, and its "cave," called the Spelaeum or Spelunca, with raised benches along the side walls for the ritual meal. Many mithraea that follow this basic plan are scattered over much of the Roman Empire's former territory, mainly where the legions were stationed along the frontiers (such as Britain). Others may be recognized by their characteristic layout, even though converted into crypts beneath Christian churches.
From the structure of the Mithraea, it is possible to surmise that worshippers gathered for a common meal seated on the benches lining the walls.
"Finally, the ubiquity of the Mithraeums’ distinctive banqueting benches implies the ubiquity of the cult meal as the liturgie ordinaire."[1]
The Mithraeum primarily functioned as an area for initiation, into which the soul descends and exits. The Mithraeum itself was arranged as an "image of the universe". Some researchers note that this movement, especially in the context of mithraic iconography (see below), seems to stem from the Neoplatonic concept that the "running" of the sun from solstice to solstice is a parallel for the movement of the soul through the universe, from pre-existence, into the body, and then beyond the physical body into an afterlife.
Similarly, the Persians call the place a cave where they introduce an initiate to the Mysteries, revealing to him the path by which souls descend and go back again. For Eubulus tells us that Zoroaster was the first to dedicate a natural cave in honour of Mithras, the creator and father of all… this cave bore for him the image of the cosmos which Mithras had created, and the things which the cave contained, by their proportionate arrangement, provided him with symbols of the elements and climates of the cosmos [trans. Arethusa edition][1]