Lake Toba (or Danau Toba in Indonesian) is a lake and former supervolcano, 100 kilometres long and 30 kilometres wide, and 505 metres at its deepest point. Toba is located in the middle of the northern part of Sumatra with a surface elevation of about 900 metres. It is the largest volcanic lake in the world. In addition it is the site of a supervolcanic eruption that occurred 69,000-77,000 years ago, a massive climate-changing event. This eruption is believed to have been the largest anywhere on Earth in the last 25 million years. According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences, killing most humans then alive and creating a population bottleneck in Central Eastern Africa and India that affected the genetic inheritance of all humans today.
The Danau Bratan lake is located inside the old caldera of the now extinct Gunung Catur volcano. The lovely area around the south and west side of the lake is known as Bedugul (see separate article).
Not far from here are the Danau Buyan and Danau Tamblingan lakes, two of the four large lakes on Bali, which can also be worth a visit. Bratan has an altitude of about 1.000 meters, the climate is quite comfortable, at nights you may even need some warm cloths.