Why does the Po Valley contain much of Italy’s flat land?

The Po Valley contains much of Italy’s flat land because it is an ancient sea gulf that has been filled by alluvium over time.1

As the most extensive and significant plain in the country, the Po valley covers more than 17,000 square miles. This vast, fertile landscape was formed and continues to be shaped by the Po River and its numerous tributaries, which carry significant sediment loads from the surrounding mountains. This ongoing process of alluvial deposition has created a complex, low-lying region that accounts for a large portion of Italy's limited flat terrain.123