These catacombs are found on land once belonging to Flavia Domitilla, wife of a first-century consul, banished to the island of Ponza for her faith. The guided visit starts with the fourth-century basilica of Saints Nerius and Achilleus, Roman soldiers martyred for proselytising, probably under Christian-hating Emperor Diocletian. The tomb of the martyrs is in the apse; throughout the church, inscriptions mark the tombs of early Christians who wanted their final resting place to be near these two fearless military men.
On a column is a representation of the martyrdom of St Achilleus with his hands tied behind his back. The ensuing galleries of burial places in the soft volcanic rock cover some 12km (7.5 miles), though you won't see all of them. The oldest part is the so-called hypogeum of the Flavi, a pagan burial ground taken over by Christians. Frescoes in varying states of preservation are found throughout.