Italy, slightly larger than Arizona, is a long peninsula shaped like a boot, surrounded on the west by the Tyrrhenian Sea and on the east by the Adriatic. It is bounded by France, Switzerland, Austria, and Slovenia to the north. The Apennine Mountains form the peninsula's backbone; the Alps form its northern boundary. The largest of its many northern lakes is Garda (143 sq mi; 370 sq km); the Po, its principal river, flows from the Alps on Italy's western border and crosses the Lombard plain to the Adriatic Sea. Several islands form part of Italy; the largest are Sicily (9,926 sq mi; 25,708 sq km) and Sardinia (9,301 sq mi; 24,090 sq km).