In ancient Roman times, no one was allowed to be buried within the walls of the city. While pagan Romans were into cremation, Christians preferred to be buried (so they could be resurrected when the time came). But land was expensive, and most Christians were poor. A few wealthy, landowning Christians allowed their land to be used as burial places.
The 40 or so known catacombs are scattered outside the ancient walls of Rome. From the first through the fifth centuries, Christians dug an estimated 375 miles of tomb-lined tunnels, with networks of galleries as many as five layers deep. The volcanic tuff that Rome sits atop — soft and easy to cut, but hardens when exposed to air — was perfect for the job. The Christians burrowed many layers deep for two reasons: to get more mileage out of the donated land, and to be near martyrs and saints already buried there. Bodies were wrapped in linen (like Christ's). Since they figured the Second Coming was imminent, there was no interest in embalming the body.
When Emperor Constantine legalized Christianity in A.D. 313, Christians had a new, interesting problem: There would be no more recently persecuted martyrs to bind them together and inspire them. Instead, the early martyrs and popes assumed more importance, and Christians began making pilgrimages to their burial places in the catacombs.
In the 800s, when barbarian invaders started ransacking the tombs, Christians moved the relics of saints and martyrs to the safety of churches in the city center. For a thousand years, the catacombs were forgotten. In early modern times, they were excavated and became part of the Romantic Age's Grand Tour of Europe.
When abandoned plates and utensils from ritual meals were found, 18th- and 19th-century Romantics guessed that persecuted Christians hid out in these candlelit galleries. The popularity of this legend grew — even though it was untrue. By the second century, more than a million people lived in Rome, and the 10,000 early Christians didn't need to camp out in the catacombs. They hid in plain view, melting into obscurity within the city itself.
The underground tunnels, while empty of bones, are rich in early Christian symbolism, which functioned as a secret language. The dove represented the soul. You'll see it quenching its thirst (worshipping), with an olive branch (at rest), or happily perched (in paradise). Peacocks, known for their purportedly "incorruptible flesh," embodied immortality. The shepherd with a lamb on his shoulders was the "good shepherd," the first portrayal of Christ as a kindly leader of his flock. The fish was used because the first letters of these words — "Jesus Christ, Son of God, Savior" — spelled "fish" in Greek. And the anchor is a cross in disguise. A second-century bishop had written on his tomb, "All who understand these things, pray for me." You'll see pictures of people praying with their hands raised up — the custom at the time.
All catacomb tours are essentially the same. Which one you visit is not important. The Catacombs of San Callisto, a.k.a. Callixtus), the official cemetery for the Christians of Rome and burial place of third-century popes, are my favorite. This was the official cemetery for Rome's early Christians and the burial place of nine third-century popes, other bishops of Rome, and various martyrs. Buy your €6 ticket and wait for your language to be called. They move lots of people quickly. If one group seems ridiculously large (over 50 people), wait for the next tour in English.
The Catacombs of San Sebastian (Sebastiano) are 300 yards farther south down the road.