Subterranean cemeteries outside the walls of Rome, which were in continuous use from the 2nd to the 6th centuries AD. The soft volcanic tufa of the region lends itself well to excavation, and when Roman burial practice shifted from cremation to inhumation in the 2nd century AD, the need for more space led to the extension of cemeteries underground. Approximately 35 catacombs are known, ranging in size from a single burial chamber (cubiculum) for the use of one family to vast multi-level networks of underground passages and cubicula. Although such cemeteries were used by the adherents of many faiths that practised inhumation, the majority can be identified as Christian, particularly in the years following the legitimization of Christianity by Emperor Constantine I in 313.