Carcere Mamertino according to a legend was the jail where St Peter and St Paul was imprisoned. They would have stayed here for nine months and could runaway thanks to the help of other prisoners converted by apostles. Thus it was transformed in the place of cult with the name San Pietro in Carcere. It consists of two levels and was the most horrible part of the ancient State prison located in the caves under the Capitoline Hill. Numerous illustrious enemies of Rome died here. The higher part of the prison was built in the 2nd century b.C. and was named "Mamertino" (medieval name). The lower part is named "Tullianum" built in 387 b.C. which originally was a water reservoir (tullus).