Lake Mälaren is the third largest lake in Sweden after Lakes Vänern and Vättern. Its surface area is 1,140 km², its average depth is 12 meters and its greatest depth is 64 m. The lake drains, from southwest to northeast, into the Baltic Sea. The two largest islands in Mälaren are Selaön (91 km²) and Svartsjölandet (79 km²). The lake was previously known as Løgrinn. The Viking settlements Birka on the island of Björkö and Hovgården on the neighbouring island Adelsö have been a UNESCO World Heritage Site since 1993, as has Drottningholm Palace on the island of Lovön.