Lake Toba is the largest volcanic lake in the world and the site of the most violent volcanic eruption of the past two million years. The lake occupies a caldera roughly 100 kilometres long and 30 kilometres wide, formed when a supervolcano collapsed after its cataclysmic eruption approximately 74,000 years ago. Samosir Island rose from the caldera floor over subsequent millennia due to continued volcanic pressure from below — making it a resurgent dome rather than a simple island.
Standing on Samosir and looking out over the dark water, it is difficult to fully comprehend the scale of the setting. The lake is 505 metres deep at its deepest point. The caldera walls rise around the island in the form of steep green escarpments. The whole thing is, geologically speaking, still an active system — small earthquakes are periodically felt, and hot springs emerge on the eastern shore of Sumatra near the lake.
Visitors experience this primarily through the landscape rather than any formal geological attraction. There is no volcano to climb; the volcano is what you are standing in. The best overview of the caldera landscape comes from the high plateau in the interior of Samosir, where the terrain levels out and you can see across to the Sumatran rim on all sides.