If you have even a slight curiosity about the dead, you will want to add the Catacombs of St. Callixtus to your touring itinerary. The catacombs are spread out over 37 acres along the old Appian Way (Via Appia). Below ground, there are five levels of graves. The funerary tunnels are about 12 miles long. Estimates suggest over 500,000 people were buried at Catacombe di San Callisto, including 16 popes from the 2nd through the 4th centuries and numerous Christian martyrs. The catacombs became inactive during the 9th century and were discovered in 1849. This Christian cemetery is part of a larger, 74 acre complex called Complesso Callistiano.
