One of the things we wanted to to at Ostia was to visit as many Mithraeums as possible. In this we succeeded nicely...and failed dismally. I'll explain in a moment, but first a brief aside is necessary. Half of my usual audience knows more about Mithras than I do. The other half has never heard of him.
Mithraism is the worship of Mithras. It was a religion roughly contemporary with Christianity and the degree to which one influenced the other has been the subject of much scholarly inquiry. Mithraism, like Christianity, was an Eastern cult, probably originating in mid First Century AD. Mithras had a miraculous birth - springing to life from solid rock - on December 25th.
Of course there were also many differences. Mithraism was practiced only by men, and was popular in the Roman Army. It had very typical iconongraphy. The classic image is Mithras wearing a Phrygian hat (think Smurf) slaying a bull in an underground setting. He had two assistants Cautes and Cautopates who wore similar head gear and carried torches. Various other elements including animals and zodiac images were common. Rites were held in underground rooms that recall the caves in the original Mithras story.
Ostia Antica has a large number of Mithraeums. 18 have been identified. I believe that in all of Rome only 8 are extant, and only one or two are open to view.
I wish I had found this SITE, which describes all the Ostian Mitraeums, prior to our visit. Alas. Well, we did the best we could.
This is the Mithraeum of the Terme del Mitra. Basically that means it is the Mithraeum associated with baths that were named after the Mithraeum. Somewhat circular language that. It is an underground site with the skylight above the central sculpture designed to let in an illuminating ray of sunshine. The arrangement is typical of Mithraeums, a central lane and benches on either side. The latter were where the devotees of Mithras would repose while consuming a ceremonial meal. From excavation evidence it seems to have actually been vegetarian fare.
A close up of the depiction of Mithras slaying the bull. This is called a Tauroctony. What you see here is a replica but the original is in the site's museum and is spectacular. If you are wondering why the later Christian inhabitants left it alone, well, they didn't. The bull's head and the head and arms of Mithras were bashed off and thrown in a nearby ditch. A Christian structure was then built over the top. Iconoclasm of this sort makes it a bit difficult to tell how many Mithraeums (or to be linguistically perfect, Mithrae) existed in Ostia. Perhaps 100? Some were probably never excavated, others were destroyed in ancient renovations, perhaps others were taken on by more devout and industrious Christians.
This is the Mithraeum of the Seven Gates. It is under a modern roof, not nearly as evocative as a pseudo cave. The mosaics and wall paintings look to be deteriorating as compared to earlier photos. Note the same arrangement of benches.
Boy did we hunt a while for this last one. It was far out in the seldom visited periphery of the site. It is the Mithraeum of Fellicissimus. It gets its name from an "ex voto" inscription in a mosaic, that indicates someone named Fellicissimus built this shrine in fulfillment of a vow. In the foreground are the twin caps of Cautes and Cautopates. The mosaics that run down the center are divided into seven panels with a final panel showing an altar. Seven was an important number in the Mithraic mysteries. It represented the seven known planets and the number of levels of initiation into the Mysteries.
The link I listed above has a map showing the locations of all the Mithraeums. We did look in on several more. But I can attest to the fact that some are very hard to see, due to indifferent maintenance, or in the case of some underground ones are closed off entirely.