The Mithreum, a place consecrated to the cult of the God Mithra worshipped in the Orient, is situated in one of the underground galleries assigned to the services of the Thermae of Caracalla. It was discovered in 1912 and proves to be the largest in Rome. It can be dated after the construction of the thermae (beginning of the third century A.D.) and consists of five chambers that communicate with the upper floor through a staircase. The main chamber consists of a vast rectangular room with cross vaults, closed by a door. 

The mosaic flooring is white with black stripes; there are two high benches on the sides, where the worshippers sat during the cerimonies. On one of the walls there is a fresco that depicts the God Mithra and a figure with a torch holding a solar disc in his left hand. 

There is a rectangular ditch at the center of the room, where the bull was probably sacrificed according to the ritual of the cult of Mithra. This chamber gives access to another room where a brick bench and a small basin with steps at its center are situated. Part of the statuary group representing Mithra in the act of killing the bull and an altar were found in fragments inside the Mithreum.