Lake Toba (Danau Toba) is the largest volcanic lake in the world and is in Sumatra, Indonesia. Samosir island within the lake is an island within the island of Sumatra.

Lake Toba is an immense volcanic lake covering an area of 1,707 km² (1,000 km² bigger than Singapore) with an island in its centre. Formed by a gigantic volcanic eruption some 70,000 years ago, it is probably the largest resurgent caldera on Earth. Some studies say it might have been associated with causing previous ice age/climate change and the largest human population bottleneck ever. Genetic estimates suggests that there were only a few thousand individuals that survived its catastrophic eruption.

Source: http://wikitravel.org/en/Lake_Toba