Blue Lake (South Australia)

The Blue Lake is in Mount Gambier, South Australia. It may be a dormant volcanic crater. It is one of four crater lakes on Mount Gambier. Over the past 30 to 40 years the water table has dropped. The Blue Lake changes colour, from grey to bright blue.
Different dates have been given for its last eruption, of 28,000 years ago, 4300 years ago, and, recently, a little before 6000 years ago. If the youngest date is correct, this could be the most recent volcanic eruption on the Australian mainland.
Blue Lake has an average depth of 72 metres, but in places reaches 77 metres deep. The crater rim measures 1,200 metres (3,937 feet) by 824 metres (2,703 feet). 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.
Major diving exploration of the lake first took place in 1985. Cave diver Peter Horne studied the temperature and visibility of the water. He discovered a fresh water sponge species and other invertebrates. 
He also discovered "The stromatolite field", hollow rock formations on the north-eastern edge at a depth of 40 m. In 2008 another dive brought up core samples from the calcite-silt covered lake bed, and measured the water temperature on the lake bed at 14 degrees Celsius (57 °F).