From a pleasant garden (its entrance is to the left of the church) overlooking Sanità, steps lead down into the two-level catacombs, with their fascinating - if deteriorated - second-century frescoes. This was a burial place rather than a hideout; early Neapolitan Christians were persecuted in a far less systematic fashion than their counterparts in Rome. After the body of San Gennaro (St Januarius) was brought here from Pozzuoli in the fifth century, it became an important place of pilgrimage. Fine arcosolia (sarcophagi carved into tufa walls and topped with arched, frescoed niches) fill the upper levels in the main ambulatory, with fifth-century mosaics and frescoes from the second century in the vestibule, one possibly portraying Adam and Eve. The lower level has an eighth-century baptismal tub and a chapel dedicated to Sant'Agrippino. Catacomb tours take 40 minutes.
Regrettably, the early Christian basilica of San Gennaro has been closed to the public for years, and is currently used as a storeroom by the hospital.