While jails and prisons were not a common feature in classical Rome (as execution, exile, and forced labor were a bit more du jour) one ancient lock-up has survived down the ages and it is now said to have held some of the Bible’s most hallowed saints.
In the time of ancient Rome (between 600 and 500 BCE) the little jail cell was known as the Tullianum and was actually constructed as a cistern for a spring in the floor of what would become one of the cells. Once the site was converted into a jail, the two cells were created on top of one another with the entrance to the lower cell being a small door in the floor of the upper cell. During their use historic sources have described the cells as dank, torturous dungeons. However inmates rarely stayed long in the jail, usually simply awaiting trial or execution.
Eventually the ancient prison took on a great deal of Christian significance (ironic since a number of Christians were imprisoned at the site) thanks to the probably incorrect linking of the place to a tale from the Bible. At some point the small jail was connected to a story of a Christian man being jailed by the Romans and making a spring rise up right in his jail cell. As the site was originally built around the spring in the floor, this is unlikely the same site, but nonetheless, pilgrims from all around began flocking to the prison, and an altar was even put in the lower cell.
In fact today the Mamertine Prison is located beneath a large church. The altar that now sits in the lower cell is decorated with an upside down cross, giving the damp dungeon an eerie air. However this inverted symbol is actually a reference to Saint Peter who was thought to have been crucified upside down.
Update as of December 2019: The lower altar is currently undergoing restoration. While the room is still accessible, all artifacts have been moved upstairs to a new museum display.