Rome's most famous underground tourist attraction, the catacombs, are outside the city walls, as ancient Roman law forbade burials within the city boundary. Of Rome's 65 known catacombs-networks of hand-dug tunnels that become massive "dormitories" for the dead-only a handful are open to the public. The catacombs of San Callisto are the largest, with 500,000 burial niches/graves.