The place where the Catacomb of San Sebastiano stands was in ancient times a deep depression, used as a pozzolana quarry and called ad catacumbas (or “near the cavities”), a name that has become synonymous with an underground cemetery.

Since the first century, the site has been intensely exploited and built: the underground galleries were reused to obtain burial niches, on the surface, however, several columbaria were built and at least two residential systems (the so-called small villa and large villa) equipped with remarkable pictorial wall decorations.

Around the middle of the second century, the bottom of the valley was buried to create a pitch, on one side of which three mausoleums were built in succession (by Clodius Hermes, by the Innocentiores, by the Ax).

A further burial of the area was made to give space to the construction of the triclia, a portico bordered by a wall on which numerous graffiti invocations addressed to the SS. Peter and Paul, who were venerated together in this place around 250.

On the site, then, the emperor Constantine (306-337) had a basilica in the shape of a Roman circus (called “circiforme”).

Meanwhile, already in the third century, the catacomb that housed the tombs of the martyrs Sebastian and Eutichio began to develop underground.