The catacombs of San Gennaro are the most important in southern Italy in terms of size and historical/artistic value. They lie on two, not directly overlapping, levels, which follow a wide, horizontal, rather than vertical, excavation pattern over a total walkable area of 5,600 square meters.

The artistic stratification is clear and impressive, showing that the rooms, which are decorated with frescoes and mosaics, date from the 2nd to the 6th century A.D. The so-called ‘lower vestibule’ is particularly striking, looking as it does like a proper underground city, and the ‘Basilica maior’, a subterranean basilica with three naves dug out of the tuff and dedicated to the ancient cult of San Gennaro, the patron saint of Naples, is also very beautiful. The basilica preserves numerous frescoes dated to the 5th-6th century A.D.