Mithraic initiates moved through seven named ranks inside artificial caves built to look far older and rougher than the cities around them.
Built to look like caves
Mithraea were typically constructed as long, narrow underground or semi-subterranean halls with benches along each side, deliberately designed to evoke a natural cave regardless of their actual urban setting, centred on a carved or painted relief of the tauroctony, Mithras slaying a bull, the religion's single most consistent and widespread iconographic image across the empire.
Seven ranks, few surviving names in full
Initiates progressed through seven grades, including ones named Raven, Persian and Lion, each apparently associated with specific symbols, planetary associations and ritual roles, though because no Mithraic scripture survives, reconstruction of what exactly each grade meant or required relies heavily on inscriptions, graffiti and the physical arrangement of surviving mithraea rather than any direct textual explanation.
Found under later cities
Well-preserved examples survive beneath later construction, including a mithraeum discovered under the Basilica of San Clemente in Rome and another excavated beneath a modern office redevelopment in London, both illustrating how thoroughly this once-widespread cult's meeting spaces were built to be enclosed, hidden and physically separate from the ordinary civic and religious life of the cities around them.