In ancient times, before the founding of the city of Rome, the Etruscans began the practice of burying their dead in underground chambers. Although the early Romans usually cremated their dead, the early Christians revived the use of catacombs because cremation was thought to deny the Christian belief in the resurrection of the body. By the second century, Christians had begun excavating extensive catacombs in the soft rock outside the city, because Roman law prohibited burial in the city limits. Many of these burial chambers were large enough to hold memorial services and celebrations of the martyrs buried there. The catacombs were probably not used for regular worship, and it’s possible that they served as hiding places during persecutions, although scholars disagree about this point.
There are forty known catacombs in Rome, most built along major roads leading to the city. The largest are named after the martyrs believed to have been buried there: St. Callixtus, St. Sebastian, and St. Priscilla. Most of them are extensive galleries and passages carved 22-65 feet below the ground, and together they total as much as 600 acres. Burials were into carved niches or sarcophagi, and often the walls were covered with murals or frescoes.
By the 10th century, the catacombs of Rome were almost entirely forgotten. They were accidently rediscovered in 1578. Today the catacombs are maintained by the Vatican, and are popular destinations for tourists and pilgrims.