Syracuse's catacombs were hewn out of a solid layer of limestone rock, which meant that vast spaces could be opened up. Built around the tomb of San Marciano, one of the earliest martyrs, the Catacombe di San Giovanni (4C-6C) are organised around a main rectilinear axis with secondary galleries leading off it. The arcosolium sepulchres (niches with arches at the top) are arranged along the walls with up to 30 spaces for bodies.