Christian graves in the Catacombs of San Sebastiano, Rome, Via Appia Antica.

The Catacombs of San Sebastiano are a hypogeum cemetery in Rome (Italy), rising along Via Appia Antica, in the Ardeatino Quarter. They are one of the very few Christian burial places that have always been accessible. One of the smallest Christian cemeteries, this has always been one of the most accessible catacombs and is thus one of the least preserved, Of the four original floors, the first is almost completely gone.

Originally all graves were closed, most by ceramic plates showing a manufacturer´s stamp, some by other lithic material. Because of looting during antiquity and early medieval times about 95 % of the gaves are open today. In those areas where tourists can visit the remaining bones were removed during recent years by the responsible authorities and buried in remore areas.

In ancient times the catacombs were simply known with the name in catacumbas, a Greek term composed by two words, katà and kymbe, literally meaning "close to the cavity". Actually, along the Appian Way, close to the cemetery, an evident dip in the ground is visible even now. Moreover, before its employment as a burial ground, the area was occupied by pozzolan mines, now placed about ten meters above the floor of the Basilica of San Sebastiano fuori le mura: these mines gave rise to a pagan cemetery, then used by Christians. The word catacumbas, through a process of extension and assimilation, was gradually used to identify all the hypogeum burial sites, thus simply called catacombs.
The underground graveyard, called di San Sebastiano just since Early Middle Ages, was formerly known (since the 3rd century) as in memoria apostolorum, a toponym referred to the presence within the catacomb, for some time, of the relics of the Apostles Peter and Paul.

more on https://en.wikipedia.org/wiki/Catacombs_of_San_Sebastiano , https://en.wikipedia.org/wiki/Catacombs_of_Rome#Catacombs_of_San_Sebastiano , https://en.wikipedia.org/wiki/Saint_Sebastian