Crypt
The underground chambers are now accessed from an external staircase by the right-hand aisle. This way passes by the excavated nymphaeum before reaching the actual crypt, which is below the far part of the central nave. On the way, you pass some enormous fluted column drums embedded in the right-hand wall. These are 90 cm across, and must have come from a large building; the nearby Temple of Diana is usually mentioned as a candidate. They were buried here to support the right hand aisle wall, apparently.
The actual crypt is T-shaped, with a pair of wide passages off the end nearest the church entrance. The walls are frescoed with grotesques by Anastasio Fontebuoni. The altar contains the alleged relics of St Prisca.
Mithraeum
From the crypt, you enter the Mithraeum via a doorway in the right hand wall and after passing through a little room and the original antechamber. At the far end is an altar niche, displaying the restored Tauroctony or the depiction of Mithras killing a bull, which was the central icon in the Mithraic decorative scheme. Below the depiction of the tauroctony in the niche is a representation of the god Saturn, lying down; the figure is made of amphorae (clay jars) covered with stucco. The graffito giving the date 202 is to the left in the niche.
