Constructed around 640–616 BC by Ancus Marcius, the Carcere Mamertino was a prison transformed into a pilgrimage site. Visit the two underground cells with an ominous past, where the enemies of Rome were tortured to death. Legend has it that St. Peter was imprisoned here and had miraculously created a stream of water to baptize the other inmates.

Try spotting the traces of medieval frescoes depicting Jesus, the Virgin Mary and St. Peter and Paul on the bare stone walls of the 7th century BC carcere (prison). Head to the lower room or Tullianum, and look out for the stone, which held the imprint of St. Peter's head when he was tossed into prison. See the sinister upside down cross hanging from a red marble and an altar with a relief of St. Peter baptizing his fellow prisoners.