The hypogeum (underground) located below the church of San Salvatore is constituted by a complex of spaces excavated in the rock at the bottom and constructed with rows of bricks alternating with rows of stone blocks in the upper part. The access to the structure is by means of an open staircase on the floor of the church that leads, trough a corridor, into two vaulted rectangular rooms (I and V); at the end of the corridor is a small circular room, with a domed roof and a square pit, that leads to two vaulted side chambers, with the bottom side apse (II and IV), and one vaulted semi-circular room (III).
The hypogeum, as well as most of its paintings and inscriptions, should be dated to the 4th century AD, but it continued to be attended, despite periods of abandonment, until modern times.

On the plastered walls of the rooms numerous paintings in black are preserved, which represent gods and heroes of the classical tradition (including Venus, Mars, Pegasus, Persephone, Nymphs, Hercules fighting the Nemean lion), other figures (male character surrounded by lions, victorious charioteers), Christian symbols (peacock, fish) and numerous boats, in one case perhaps a galleon of the 17th century AD. These representations are related to a cult connected with the salutary waters, that was certainly practiced in the chapel; there are also numerous Latin inscriptions, a Greek alphabet and one Arabic inscription (16th-7th centuries AD).