<P> Blue Lake is thought to be of an average depth of 72 metres (236 ft), but in places reaches 75 metres (246 ft) deep . The crater rim measures 1,200 metres (3,900 ft) by 824 metres (2,703 ft), however, the lake itself measures 1,087 metres (3,566 ft) by 657 metres (2,156 ft). The bottom of the lake is 30 metres (98 ft) below the level of the main street of the nearby town . The Blue Lake supplies the town with drinking water . </P> <P> Bathymetric surveys located the deepest point in the lake at 77 metres (253 ft) 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 fresh water 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 40 metres (130 ft). 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 14 ° C (57 ° F). </P> <P> 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 it is generally considered likely that it revolves around the warming of the surface layers of the lake during the summer months to around 20 ° C (68 ° F), causing calcium carbonate to precipitate out of the solution and enabling micro-crystallites 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 . </P> <P> 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: </P>

Why is the blue lake blue in mt gambier
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