Worshippers of Mithras had a complex system of seven grades of initiation and communal ritual meals. Initiates called themselves "syndexioi", those "united by the handshake". They met in underground temples, now called "mithraea" (singular "mithraeum"), which survive in large numbers. The cult appears to have had its centre in Rome, and was popular throughout the western half of the empire, as far south as Roman Africa and Numidia, as far north as Roman Britain, and to a lesser extent in Roman Syria in the east.