Lake Toba is both Indonesia’s largest lake and the world’s largest volcanic lake. It occupies the caldera of the Toba supervolcano in North Sumatra, where the immense basin stretches about 100 kilometers long and 30 kilometers wide.
The lake formed after a colossal explosive eruption roughly 74,000 years ago, generally classified as magnitude 8 on the Volcanic Explosivity Index. The eruption produced enormous volumes of ash and pyroclastic material and left a giant caldera that later filled with water. Samosir Island is a prominent resurgent feature within the lake, rising from the caldera’s center.
Lake Toba covers about 1,130 square kilometers and reaches depths exceeding 500 meters. Its outflow is the Asahan River, which eventually reaches the Strait of Malacca. Lake Matano is deeper, and Lake Maninjau is also volcanic, but neither matches Toba’s combination of national size and global volcanic-lake status. The “largest volcanic lake” description refers to surface area, not maximum depth.