Tower Hill has always been a part of Koroit and in the town's earliest days the lake within the Tower Hill crater was the source of the town's drinking water. Tower Hill is an extinct volcano formed at least 30,000 years ago when a hot rising basaltic magma came into contact with the subterranean water table. The violent explosion that followed created the funnel-shaped crater (later filled by a lake) and the islands seen today. It is one in a line of more than 30 volcanoes that stretch from Colac to the East to Mount Gambier in South Australia. Artefacts found in the volcanic ash layers show that Aborigines were living in the area at the time of the eruption. This area including Tower Hill is part of the UNESCO-endorsed Kanawinka Geopark.