The gradual subsidence of the Po Valley (including that of Venice) and the folding of the mountains of eastern Italy have been investigated using seismic wave analysis of the Apennine Subduction System. Along the Adriatic side of Italy the floor of the Adriatic Sea, referred to as the Adriatic lithosphere or the Adriatic plate, terms whose precise meaning is the subject of ongoing research, is dipping under the slab on which the Apennines have been folded by compressional forces.