A vault or subterranean chamber, especially one beneath the main floor of a church or other building and used as a burial place. In the catacombs, it was a tomb in which a number of bodies were interred together. Early Christians called their catacombs crypts; and, when churches came to be erected over the tombs of saints and martyrs, subterranean chapels, known as crypts or confessiones, were built around the actual tomb. The most famous of these was St. Peter's, built over the circus of Nero, the site of St. Peter's martyrdom. By the time of Roman emperor Constantine the Great (306-337), the crypt was considered a normal part of a church building.