For the most part, mithraea tend to be small, externally undistinguished, and cheaply constructed; the cult generally preferring to create a new centre rather than expand an existing one. The mithraeum represented the cave to which Mithras carried and then killed the bull; and where stone vaulting could not be afforded, the effect would be imitated with lath and plaster. They are commonly located close to springs or streams; fresh water appears to have been required for some Mithraic rituals, and a basin is often incorporated into the structure. There is usually a narthex or ante-chamber at the entrance, and often other ancillary rooms for storage and the preparation of food. The extant mithraea present us with actual physical remains of the architectural structures of the sacred spaces of the Mithraic cult. Mithraeum is a modern coinage and mithraists referred to their sacred structures as "speleum" or "antrum" (cave), "crypta" (underground hallway or corridor), "fanum" (sacred or holy place), or even "templum" (a temple or a sacred space).