the mithraeum was either an adapted natural cave or cavern, or a building imitating a cave. when possible, the mithraeum was constructed within or below an existing building, such as the mithraeum found beneath basilica of san clemente in rome. while a majority of mithraea are underground, some feature open holes in the ceiling to allow some light in, perhaps to relate to the connection of the universe and the passing of time. the site of a mithraeum may also be identified by its singular entrance or vestibule, which stands opposite from an apse-shaped wall in which a pedestal altar at the back stood, 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, particularly where the legions were stationed along the frontiers (such as britain). others may be recognized by their characteristic layout, even though converted as crypts beneath christian churches.