From archaeology.org: Martyrs or Imperial Guard?.

When a sinkhole opened up after a pipe broke underneath the convent and school of the Instituto Sacra Famiglia on Rome’s Via Casilina, the sisters there received a surprise — about 1,200 surprises, in fact. The partial collapse of the building’s foundation revealed five large chambers in which the remains of more than a thousand individuals had been interred almost simultaneously sometime at the beginning of the third century A.D.

Perhaps equally surprising is the location in which they were found. The convent under which the burial chambers are located sits atop the vast catacomb complex of San Pietro and Marcellinus. With three distinct gallery levels, the deepest of which is 36 feet (11m) below the surface, it is one of the largest such burial complexes in the city.

But the newly discovered burial chambers pre-date the extensive catacomb complex, which was believed to have been used by Christians from the mid-third century A.D. with permission from the emperor Gallienus who was anxious to make peace with them after the savage persecution they suffered at the hands his father, Valerian. And although the famed archaeologist Giovanni Battista de Rossi explored and recorded the catacomb at the end of the nineteenth century, there is no indication that he ever even knew of the presence of these chambers. [continue]