Discovered in 1912 by Ettore Ghislanzoni, the mithraeum at the baths is considered the largest documented gathering space for the worshippers of Mithra, the Persian god in vogue with the military and mostly lower class men, in the 2nd and 3rd centuries AD. The mithraeum was approximately long and wide with a cross-vaulted ceiling. It can be only roughly dated by the two main events associated with the baths: the mithraeum was created after the complex was completed circa AD 217 and it was probably no longer in use when the aqueduct supplying the complex was cut in the 530s.