At the foot of the Campidoglio, 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 the bare stone walls you can just make out early Christian frescoes depicting Jesus and Sts Peter and Paul.
If you just want to nip in and have a look, get the €3 ticket, otherwise guided multimedia tours take about half an hour.
