Catacombs of San Callisto

Twenty metres below the Appian Way, there are twenty kilometres of tunnels carved from soft volcanic tufa rock, stacked four levels deep, containing the remains of an estimated half a million people. The Catacombs of San Callisto are the largest and most important of Rome's sixty-plus catacombs, and they served as the official cemetery of the Church of Rome in the third century. Sixteen popes are buried down here.

The common misconception is that early Christians hid in the catacombs to escape persecution. They did not. Roman law considered burial places sacred and inviolable — even the authorities would not enter. The catacombs were not hiding places but legal burial grounds, chosen because Roman law prohibited burial within the city walls and land outside Rome was expensive. Underground burial solved the space problem. The tunnel walls were carved with shelf-like niches called loculi, each sealed with a marble slab or tiles, stacked five or six high.

The art down here is some of the earliest Christian art in existence. You will see third-century frescoes of Jonah and the whale, the Good Shepherd, and fish symbols (ichthys) — visual codes that identified fellow believers when Christianity was still an underground (pun intended) movement. The fish symbol worked because the Greek word for fish, ichthys, is an acronym for "Jesus Christ, God's Son, Saviour." This was effectively an early encryption system.

The temperature down here is a constant thirteen to sixteen degrees Celsius year-round, which is why the frescoes and inscriptions have survived so well. Guided tours are the only way in, and they last about thirty minutes. The tunnels are narrow, the ceilings low, and the deeper levels have a silence that is genuinely disorienting. You are walking through a city of the dead that makes the one above look temporary.