The Indonesian lake filling the caldera of a supervolcano on Sumatra is Lake Toba.
Lake Toba occupies the Toba Caldera in North Sumatra and is Indonesia’s largest lake. It stretches roughly 100 kilometers long and 30 kilometers wide, with a maximum depth of about 505 meters. Samosir Island rises near its center on a resurgent dome formed after the caldera-forming eruption.
The Toba supereruption occurred about 74,000 years ago and is classified as a magnitude-8 event on the Volcanic Explosivity Index. The eruption released enormous volumes of ash and pyroclastic material; the collapsing volcanic system then created a vast depression that filled with water to become the lake.
Lake Toba is sometimes described simply as a volcanic lake, but its deeper geological identity is a lake inside a giant caldera. The eruption’s possible effects on global climate and human populations remain subjects of scientific debate. Toba Caldera was also recognized as a UNESCO Global Geopark in 2020, connecting its geology with Batak cultural heritage and dramatic highland scenery.