Mount Gambier's urban area is located mainly along the northern slopes and plain of a maar volcano of the same name, Mount Gambier. Comprising several craters, it is part of the Newer Volcanics Province complex of volcanoes. One of these contains a huge lake of high-quality artesian drinking water which changes colour with the seasons. In winter, it is a steel grey and then changes to a spectacular cobalt blue in the summer, giving rise to its name, Blue Lake. This deep lake also accommodates a range of unusual aquatic flora and fauna, in particular fields of large stromatolites. There are several other craters in the city including Valley Lake and the Leg of Mutton Lake. The region surrounding the city also includes other volcanic features such as Mount Schank, along with many karst features such as water-filled caves, cenotès and sinkholes.