The Sidon Mithraeum is an underground Roman-era sanctuary discovered during excavations in the heart of the old town. Sanctuaries to Mithras — the Persian-derived mystery cult popular with Roman soldiers and traders from the 1st to 4th centuries AD — were typically built underground, simulating the cave in which Mithras was said to have slain the bull. The Sidon example fits the type: a small vaulted chamber with side benches where the cult's adherents would have shared ritual meals.
Access depends on the archaeological authorities and is not always possible, but the site is significant as one of relatively few Mithraea found in the Levant. Check with the Audi Soap Museum or local tourist office for current visiting arrangements.