Catacombs were subterranean cemeteries that served a double purpose: that of burial for Christians who had died or been martyred, and a place to which Christians could resort to conduct devotions, administer sacraments, and celebrate the Eucharist in secrecy because of persecution.
It was customary to celebrate the mystery of the Eucharist, the Mass, using the top of the tomb for an altar surface.
Many catacombs have been excavated, the majority located along the Via Appia and the Via Ardeatina (two ancient Roman roads).
The catacombs were dug by burrowing into the soft stone of hill areas. The practice was to excavate a stairway and then open a narrow gallery. Within this gallery were tombs cut into the wall, capable of holding from one to three bodies. These tombs were sealed with a slab of stone when the bodies had been placed within. When the gallery became full, it was the usual practice to cut lateral galleries to expand capacity.
In the fourth century, after the end of the persecutions, the catacombs became places of pilgrimage and have remained so until the present.
The catacombs are notable for the testimony of art and practice that they supply to modern ages. Chief of these testimonies are that of the sacramental religion observed by these early Christians and the purity of the spiritual truth that is visible. These fact are deduced from the art, the carvings, artifacts, and inscriptions with which the catacombs contain. (7:26)