The province of North Sumatra stretches across the island of Sumatra between the Indian Ocean and the Strait of Malacca. It borders Aceh province on the northwest and Riau and West Sumatra provinces in the southeast. It has an area of 72,981  km2. The province contains a broad, low plain along the Strait of Malacca on which the provincial capital, Medan, is located. In the south and west, the land rises to the mountain range that runs the length of Sumatra; the mountains here are dominated by Lake Toba, formed from the caldera of an ancient volcano. Several large islands in the Indian Ocean off the west coast of Sumatra are currently part of North Sumatra, most notably Nias Island and the Batu Islands.