Lake Toba on the island of Sumatra is an immense volcanic lake about 100 kilometers long and 30 kilometers wide. Formed by a gigantic volcanic eruption some 70,000 years ago, it is the largest resurgent caldera on Earth. Genetic estimates suggests that there were only a few thousand humans that survived the catastrophe. The island in the middle – Pulau Samosir – is the largest island within an island and contains two lakes. Besides visiting “a lake on an island within a lake on an island” tourist also come here to kick back and relax and swim in the volcanically warmed waters.