Dug between the 2nd and 5th centuries, this vast network of moody tunnels was known as the Queen of Catacombs. It was an important early Christian burial site and numerous martyrs and popes are buried in the 40,000 tombs and chambers that line the 13km of tunnels.
Tours (35 minutes; alternating in English and Italian throughout the day) take in a decorated 3rd-century-AD Greek chapel and a rough fresco of the Virgin Mary with the baby Jesus on her lap. Dating from around AD 230, this is thought to be the oldest existing image of the Madonna.