The lowest levels of the present basilica contain remnants of the foundation of a possibly republican era building that might have been destroyed in the Great Fire of 64. An industrial building – probably the imperial mint of Rome from the late 1st century A.D. onwards (because a similar building is represented on a 16th-century drawing of a fragment of the Severan marble plan of the city), was built or remodelled on the same site during the Flavian period. Shortly after an "insula", or apartment block, was also built . It was separated from the industrial building by a narrow alleyway. About a hundred years later (c. 200) a "mithraeum," a sanctuary of the cult of Mithras, was built in the courtyard of the "insula." The main cult room (the "speleum", "cave"), which is about 9.6m long and 6m wide, was discovered in 1867 but could not be investigated until 1914 due to lack of drainage. The "exedra", the shallow apse at the far end of the low vaulted space, was trimmed with pumice to render it more cave-like.