Vättern (vĕˈtərn) [key], lake, 733 sq mi (1,898 sq km), c.80 mi (130 km) long and up to 20 mi (32 km) wide, S central Sweden, drained by the Motala Ström E into the Baltic Sea. It is the second largest lake in Sweden. The Göta Canal crosses the northern part of the deep lake. Visingsö (c.10 sq mi/25 sq km) is the largest island in the Vättern; a prehistoric burial mound is there. Motala and Jönköping are the principal cities on the lake.