The key evidence of the difference is the geologic behavior of the Po Valley in northern Italy. Compressional forces have been acting from north to south in the Alps and from south to north in the Apennines, but instead of being squeezed into mountains the valley has been subsiding at 1 to per year since about 25 mya, before the Apennines It is now known to be not an erosional feature but is a filled portion of the Adriatic Trench, called the Adriatic foredeep after its function as a subduction zone was discovered. The Alps and the Apennines were always separated by this trench and were never part of the same system.