Hidden beneath the 16th-century Chiesa di San Giuseppe dei Falegnami, the Mamertine Prison was ancient Rome's maximum-security jail. St Peter did time here and while imprisoned, supposedly created a miraculous stream of water to baptise his jailers.
On its bare stone walls, you can make out traces of medieval frescoes depicting Jesus, the Virgin Mary and Sts Peter and Paul.
Visits take in a small street-level museum, a two-room affair with artefacts excavated on the site, while tablets provide graphic 3D illustration of how the complex developed over time.
From the museum, stairs lead down to the prison below, which is set on two levels: the 7th-century BC carcere (prison) proper with its frescoed walls, and beneath that a dungeon known as the Tullianum. This chilling stone cell is where enemies of the state were thrown and left to die.