Blue Lake (South Australia)
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 (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 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 , but in places reaches deep (but some unconfirmed values mention a maximum depth due to a natural cave section). The crater rim measures , but the lake itself measures . The surface of the lake is below the level of the main street of the nearby town. The Blue Lake supplies the town with drinking water. Studies. Bathymetric surveys located the deepest point in the  lake at in 1967. Major diving exploration of the lake first occurred in 1985. Cave diver Peter Horne conducted temperature and visibility studies and made discoveries of a freshwater sponge species and other invertebrates. This exploration also discovered the Stromatolite Field, a collection of hollow rock formations that are found along the north-eastern perimeter down to a depth of . In 2008, permission was granted by SA Water for another diving exploration of the central and deepest parts of the lake. On this dive, core samples from the calcite-silt covered lake bed were collected where water temperature drops to  . Annual colour change. During December to March, the lake turns to a vibrant cobalt blue colour, returning to a colder steel grey colour for April to November. The exact cause of this phenomenon is still a matter of conjecture, but likely it involves the warming of the surface layers of the lake during the summer to around , causing calcium carbonate to precipitate out of the solution and enabling microcrystallites of calcium carbonate to form. This results in scatter of the blue wavelengths of sunlight. During winter, the lake becomes well mixed, and recent research indicates that during this phase of  the colour cycle, the lake is somewhat murkier due to the redistribution of tannins and calcium carbonate particles throughout the lake. Solar elevation has also been found to influence the perceived colour of the lake. The movement of planktonic life forms within the lake during the seasons and during the day may additionally play a part in the colour change. Gordon's Leap. In July 1865, Adam Lindsay Gordon performed the daring riding feat known as Gordon's Leap on the edge of the Blue Lake. A commemorative obelisk erected there has an inscription which reads: This obelisk was erected  as a memorial to the famous Australian poet. From near this spot in July, 1865, Gordon made his famed leap on horseback over an old post and rail guard fence onto a narrow ledge overlooking the Blue Lake and jumped back again onto the roadway. The foundation stone of the Gordon Memorial Obelisk was laid on 8th July 1887