The geography of Indonesia is dominated by volcanoes that are formed due to subduction zones between the Eurasian plate and the Indo-Australian plate. Some of the volcanoes are notable for their eruptions, for instance, Krakatau for its global effects in 1883, Lake Toba for its supervolcanic eruption estimated to have occurred 74,000 Before Present which was responsible for six years of volcanic winter, and Mount Tambora for the most violent eruption in recorded history in 1815.
Mount Sempu in the northern arm of Sulawesi, Indonesia, contains a 3 km wide of caldera. A maar, called Kawah Masem, was formed in the southwest of the caldera and contains a crater lake. Sulfur deposits have been extracted from the maar since 1938. Historical records, however, are unknown from the volcano.