Up to one hundred years ago it was widely believed that the actual Basilica of San Clemente was the one to which S. Jerome was referring in 392.
But it was not. In 1857, Father Joseph Mullooly, then Prior of San Clemente, began some excavation work under the basilica during which he discovered an original basilica of the IV century, partially preserved, that was destroyed in the fire of the Normans in 1084 and that was filled with stones to build the new church on top of it, using part of the original architectural elements.
Immediately below, to a even lower level, which still remains largely unexplored, another construcion was found, that dates back to the Republican period, probably the II century BC, that includes a paleo-Christian place of cult and a temple dedicated to the god Mithras of second century (Mitreo), a Roman house and an industrial building.
Nowhere else of Rome can give such a clear idea of the stratification and the culture of the city as thousands of years of this impressive church.