Sanctity of tombs guaranteed to all creeds alike.--The Christians' preference for underground cemeteries not due to fear at first.--Origin and cause of the first persecutions.--The attitude of Trajan towards the Christians, and its results.--The persecution of Diocletian.--The history of the early Christians illustrated by their graves.--The tombs of the first century.--The catacombs.--How they were named.--The security they offered against attack.--Their enormous extent.--Their gradual abandonment in the fourth century.--Open-air cemeteries developed in proportion.--The Goths in Rome.--Their pillage of the catacombs.--Thereafter burial within the city walls became common.--The translation of the bodies of martyrs.--Pilgrims and their itineraries.--The catacombs neglected from the ninth to the sixteenth century.--Their discovery in 1578.--Their wanton treatment by scholars of that time.--Artistic treasures found in them.--The catacombs of Generosa.--The story of Simplicius, Faustina and Viatrix.--The cemetery of Domitilla.--The Christian Flavii buried there.--The basilica of Nereus, Achilleus and Petronilla.--The tomb of Ampliatus.--Was this S. Paul's friend?--The cemetery 'ad catacumbas."--The translation of the bodies of SS. Peter and Paul.--The types of the Saviour in early art.--The cemetery of Cyriaca.--Discoveries made there.--Inscriptions and works of art.--The cemetery "ad duas Lauros."--Frescoes in it.--The symbolic supper.--The discoveries of Monsignor Wilpert.--The Academy of Pomponio Leto.