The Blue Lake is a large monomictic crater lake located in a dormant volcanic maar associated with the Mount Gambier maar complex. The lake is situated near in the Limestone Coast region of South Australia and is one of four crater lakes on Mount Gambier maar. Of the four lakes, only two remain, as the other two 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 4,300 years ago, of 28,000 years ago, and, most recently, a little before 6,000 years ago. If the youngest 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 of 72m, but in places reaches 75m deep. The crater rim measures 1200m by 824m, however, the lake itself measures 1087m by 657m. The bottom of the lake is 30m below the level of the main street of the nearby town. The Blue Lake supplies the town with drinking water, providing approximately 3500ML per year from its 36000ML store.