La Stanza Gialla

Below the church of Santa Prisca is located an ancient Roman building complex. A mithraeum was accidentally discovered in 1934 by the Augustinian priests in their attempt to find the original domus ecclesiae. Two decades later, in 1952-1959 and 1964-1966, the whole site was properly investigated by two archaeologists of the Istituto Storico Olandese, M.J. Vermaseren and C.C. van Essen.
The site presents a complex aggregation of subterranean structures having different functions and belonging to different chronological phases. Large peperino and tufa ashlar walls, built in the Late Republican period and subsequently restored through Late Antiquity, are located beneath the garden on the South side of the church. The structures under the church are of Imperial date and belong to domestic quarters. Some rooms, belonging to a domus of the Julio-Claudian period (first half of the 1st century AD), bear evidence of later re--workings.

By the Trajanic period (beginning of the 2nd century AD), the building was extensively re-modeled and these rooms were incorporated in the substructures of a new domus, which included a large apsed room covered by a barrel vault and a three-sides cryptoporticus. It has been suggested this could be what remains of the house of L. Licinius Sura, who is said by the ancient sources to have lived in the area.

In the 3rd century AD, the mithraeum was built inside the cryptoporticus of the domus. Finally, in the mid 3rd century AD was built a massive brick wall that cut through the mithraeum and continued beyond the limit of the excavated area.

Nowadays, the archaeological site is accessible through an underground apsed room built in the Trajanic period. On the floor, a mosaic made of large black and white tesserae is still visible. Sculptural and architectural marble fragments – part of the excavation's findings – are displayed on the walls. The apse was used in the Middle Ages as oil mill: millstones and majolica bowls, belonging to a 14th century press, are still in situ.

An opening on the side of this room leads into a narrow corridor, where more architectural fragments and inscriptions are placed on the left wall. On the opposite side, two fluted columns in peperino stone have been re-used in the foundation of the church: considering their dimensions, it is likely that they originally belonged to a temple or a large public building.

The corridor ends into the crypt of the church, from which it is possible to access the mithraeum through a doorway on the right.