Lake Toba. Landsat 8 satellite image of Lake Toba volcano, Sumatra, Indonesia. This lake is 100 kilometres long and 30 kilometres wide. It was formed when the land collapsed following a volcanic eruption. The island at centre, Samosir Island, is a resurgent dome resulting from the refilling of the underground magma chamber. An eruption of Toba 74,000 years ago is thought to have been the largest volcanic eruption ever, causing a volcanic winter that lowered global temperatures for several years, and wiped out a large proportion of the human population. Image obtained by Landsat 8's Operational Land Imager (OLI), in May 2013.