Among the southern foothills of the Alps in the north of the mainland are the Italian lakes. Below them the River Po runs west-east across the fertile Lombardy Plain to the peninsula itself. To their west are the hills and plains of Tuscany; further south the Tiber flows across the Pontine Marshes to the Tyrrhenian Sea. Further south still the coastal plain is enriched by the debris of Vesuvius and the climate becomes warmer. To the south is Calabria, where the mountains fall steeply to the sea and in summer the land bakes brown. Eastward, stretches a wide and arid limestone plain.