The catacomb of Saints Marcellino and Pietro are located by the third mile of the ancient via Labicana, now via Casilina.

In ancient times, a toponym was called ad duas lauros (“at the two laurels”) which indicated a vast property of the emperor in addition to the cemetery area. The laurels, in fact, were traditionally shrubs placed at the entrance imperial lands.

In this place, where the necropolis of the Equites Singulares Augusti, guard on horseback of the emperor had existed since the 2nd century, the Christian catacomb was installed in the second half of the 3rd century, which hosted the bodies of numerous martyrs of Diocletian's persecution: first of all those of the Saints Marcellino and Pietro, who give the name to the catacomb.

The catacomb preserves a vast patrimony of paintings, datable to the third and fourth centuries, partly recently restored with laser technology. In the Constantinian era, the monumental complex that stood above the ground of the catacomb was erected, consisting of a large basilica in the shape of a Roman circus (called “circiforme”) connected to a mausoleum, probably built by Constantine for himself, but later destined to house the burial of his mother, Augusta Elena.

The remains of St. Helen were kept in a large red porphyry sarcophagus, which today is exposed in the Vatican Museums.