Outside the walls of Naples are the catacombs, the ancient burial grounds for the old Neapolis population. Neapolis is the ancient greek name for the city, which eventually morphed into Napoli, or Naples.
There are four known catacombs outside the city, two can be visited, the Catacombs of San Gennaro and the Catacombs of San Gaudioso. However, once you enter you quickly discover, that with some minor exceptions, there are no bodies here. In the former case they were exhumed by the French back in the 1700s and repatriated, in the latter they were moved to one of the other catacombs that currently cannot be visited.
Catacombs of San Gennaro
About a mile from the old historic district off the Corso Amedeo di Savoia are the Catacombs of San Gennaro. The largest and least morbid of the two, this catacomb has an expansive underground and very interesting history. Since a guided tour is the only option possible, you learn all the details of the historical past of this catacomb, and some of the more perculiar fact (if you are willing to ask, e.g. where did all the bodies go – back to France). Since all catacombs, more or less look the same, here is a slide show.
Catacombs of San Gaudioso
Along the same road, about a fifteen minute walk back toward the city center, are the catacombs of San Gaudioso. These are underneath the Basilica di Santa Maria della Sanità and its entrance is actually in the Basilica. This tour actually contains skeletons, bones and sarcophagi buried in the walls, plastered over with their skulls sticking out and broken off (see show for examples).