Mount Tondano in the province of North Sulawesi, Sulawesi, Indonesia, has a 20 × 30 km wide caldera which was formed in the Late Miocene or Early Pliocene by a massive eruption. Post caldera activity includes pyroclastic cones, obsidian flows and geothermal areas in the caldera area. Lake Tondano lies in the east side of the caldera. The 5 km long and 3.5 km wide ellipsoidal lies entirely within the Tondano Caldera, and formed from a large eruption of an older Somma volcano.