The Blue Lake in Mount Gambier, South Australia is a large monomictic lake located in an extinct volcanic maar associated with the Mount Gambier maar complex. It is one of four crater lakes on Mount Gambier. Of the four lakes, only two remain, as the other two (Leg of Mutton and Brown) have dried up over the past 30 to 40 years as the water table has dropped.
Conflicting dates have been estimated for its last eruption, of 28,000 years ago and 4300 years ago. If the more recent date is correct, this could be the most recent volcanic eruption on the Australian mainland.
Blue Lake is thought to be of an average depth 72 metres, but in places reaches 75 metres deep (250 feet). The crater rim measures 1,200 metres (3,937 feet) by 824 metres (2,703 feet), however, the lake itself measures 1,087 metres (3,566 feet) by 657 metres (2,155 feet). The bottom of the lake is 30 metres (100 feet) below the level of the main street of the nearby town. The Blue Lake supplies the town with drinking water, providing approximately 3,500 megalitres (2,800 acre·ft) per year from its 36,000 ML (29,000 acre·ft) store.