The Catacombs of San Gennaro are arranged on two levels that are not superimposed, both characterized by much larger spaces than the more famous Roman catacombs. This is due to the workability and strength of the tuff. The original nucleus of the Catacombs of San Gennaro dates back to the 2nd century AD. It was probably the tomb of a noble family, who then provided spaces for the Christian community.
The expansion began in the 4th century AD following the deposition of the remains of St. Agrippinus, the first patron of Naples, to the underground basilica dedicated to him. It consists of a single nave dug out of the tuff, which still preserves a bishop's chair carved into the rock and the altar with an opening, in which the faithful could see and touch the saint's tomb.