The Catacombs of San Callisto are the largest and most important early Christian burial site in Rome, spanning 20 kilometers of tunnels at 20 meters depth. They contain the remains of 16 popes and hundreds of thousands of martyrs. Guided tours reveal ancient frescoes, inscriptions, and sacred crypts including the Crypt of the Popes. A haunting yet fascinating place to learn about early Christian history and burial practices beneath the Appian Way.