Roadside tombs. Since the earliest times the common custom was to bury the dead outside the pomerium, the sacred walls of the city. The first few miles of Roman roads were therefore usually flanked by necropolises, distinguished by social status and diversity of funeral rites.
Indeed burial and cremation were practiced concurrently, one or the other prevailing according to the fashion. Under the Republic cremation was prevalent so they constructed columbariums (buildings housing thousands of funerary urns) and altars with the ashes of the ashes of the deceased.
Conversely, in the Imperial times the practice of burial became more common, resulting in the spread of catacombs. The use of underground cemeteries was a direct response to the practice of burial, since interment required more space than cremations.
From the 4th century the catacombs became almost exclusively Christian, following the conversion of the Roman people.