The Mamertine Prison, which stands in the Roman Forum, was the main gaol in Ancient Rome. The name 'Mamertino' is mediaeval; the prison was formerly called 'Tullianum', a word derived from 'tullus' or water spring, because it actually contains a spring, and was originally a reservoir. It was converted into a state security prison during the period of the Republic.

It consists of two chambers currently linked by a mediaeval staircase: the lower floor, the prison itself, of semicircular shape, is built with thick stone blocks. The Roman historian Sallust (86-34 BC) described it briefly thus: 'sunk about twelve feet under ground, walls secure it on every side, and over it is a vaulted roof connected with stone arches; but its appearance is disgusting and horrible, by reason of the filth, darkness, and stench.'

Originally its only opening was a hatch in the ceiling which communicated with the chamber above, and through which the prisoners were thrown. The lower part in its turn was linked by a passage to the Great Sewer.

This gaol held what today we would call political prisoners such as Jugurtha and Vercingetorix. The accomplices of Catiline were executed there. Its most famous inmates were the two apostles Peter and Paul.

Against the wall there is a small altar with a carved altarpiece representing the baptism of the prison wardens Processus and Martianus - an event recorded by tradition - and of forty-seven prisoners. On the left of the altar lies the place where Saints Peter and Paul are said to have awaited martyrdom while chained to a small column. Following this tradition, the chapel has been called 'San Pietro in Carcere' (St Peter in Prison) from the 16th century.