Mount Gambier is roughly 480 kilometres south east of Adelaide and is close to the city also named Mount Gambier. The Mount Gambier Volcanic Complex is a state heritage area that consists of four crater lakes which are: Blue Lake, Valley Lake, Leg of Mutton Lake and Browne's Lake. It is a volcanic complex that has become dormant, with its last eruption a minimum of 4 300 years ago. This phreatic eruption formed the craters of Blue Lake and Valley Lake and was caused by the mixing of groundwater and magma that were in close proximity. It was formed over 5 000 years ago, making it the youngest group of volcanoes in Australia. This is a rough estimate of the volcanoes' age, as the dating of this area is from a range of samples and methods. These samples are also often from swamp deposits that are younger than the volcanoes themselves.