The Catacombs of Saint Callixtus are some of the many smaller catacombs scattered around the Eternal City. Located on the Appian Way, the catacombs are famous for containing the Crypt of the Popes, which, as the name suggests, once contained the tombs of several popes from the 2nd to 4th centuries.
The Mamertine Prison holds over 3000 years worth of treasures and is one of Rome's most interesting and important monuments from the Roman Republic. It's located below the Church of St. Joseph of the Carpenters and offers one of Rome's best views of the Ancient Forum.