Catacombs are underground burial chambers, typically connected with a network of tunnels. Several cultures have a history of using these chambers to bury their dead, although they are most closely associated with the early Christian church today. Several sites around the world have extensive catacombs that are sometimes open for visitors, including the original Roman Catacombs and the Catacombs of Paris.
From a purely practical point of view, catacombs are an excellent option for disposing of the dead respectfully. Because they are underground, they do not take up valuable real estate above the earth, and their depth usually ensures that the dead will not contaminate the water supply or reemerge in periods of flooding. The chambers can also be expanded almost endlessly, in theory, to accommodate additional dead, and in fact many show signs of just that, with various chambers and tunnels being added on over the centuries.
People have been burying their dead in caves for thousands of years, but true catacombs — as in artificially constructed networks of tunnels, galleries, and niches — appear to have been constructed in the second century, in Rome. Originally, they were designed as temporary holding facilities for the bodies of Christian martyrs, and eventually they expanded. The Roman catacombs arose in response to a number of factors. Burials were forbidden in the city limits of Rome, for one thing, so these chambers could circumvent the law, and they were also useful for religious minorities like the early Christians who might otherwise be disrupted while they cared for their dead.
Historically, catacombs were used to accommodate the dead in coffins, shrouds, sarcophagi, or urns, and they were also used for memorial ceremonies. The burial ceremony might be held in the catacomb, for example, as would subsequent annual memorials and funerary feasts. Wealthy families might have a section of the chambers all to themselves, allowing them to bury each other among their ancestors.
Many are located under churches, emphasizing their connection with the early practice of Christianity. While the practice of using catacombs is less wide-spread than it once was, some monastic sects still use them for their dead, and researchers on the early church often like to visit these chambers to learn more about burial traditions in the history of Christianity. Catacombs can also reveal interesting information about the people inside of them, as many niches are marked with plaques which contain details about the life and dead of the people they contain.