In 1949 the Dutch geologist van Bemmelen reported that Lake Toba was surrounded by a layer of ignimbrite rocks, and was a large volcanic caldera. Later researchers found rhyolite ash similar to that in the ignimbrite around Toba in Malaysia and India, 3000km away. Oceanographers discovered Toba ash on the floor of the eastern Indian Ocean and the Bay of Bengal.
The Toba eruption, dated at 75,000 years ago, was the most recent eruption of a "supervolcano." Bill Rose and Craig Chesner of Michigan Technological University deduced that the total amount of erupted material was about 2800km3 -- around 800km3 of ignimbrite that flowed over the ground and around 2000km3 that fell as ash, with the wind blowing most of it to the west. Such a huge eruption probably lasted nearly two weeks. Very few plants or animals in Indonesia would have survived, and it is possible that the eruption caused a planet-wide die-off. There is some controversial evidence, based on mitochondrial DNA, that the human race was reduced to only a few thousand individuals by the Toba eruption.
A large area collapsed after the ejection of that amount of subsurface material, forming a caldera, which filled with water creating Lake Toba. Later, the floor of the caldera uplifted to form Samosir, a large island in the lake. Such uplifts are common in very large calderas, apparently due to the upward pressure of unerupted magma. Toba is probably the largest resurgent caldera on Earth.