Blue Lake, Mount Gambier

A volcanic crater lake on the edge of Mount Gambier that turns a brilliant cobalt blue from about November to March, then fades to a steely grey for the winter. Scientists put the colour change down to the warming of the surface water and changes in the calcium carbonate in it. The lake is also the city's water supply.

Walk or drive the 3.6 km rim road for views from every side. The Aquifer Tour takes a lift down the old pumping station shaft to a glass panel close to the water. The bigger loop around the whole extinct volcano, past the Valley Lake, Mount Gambier's lookout tower and picnic parklands, is about 6 to 7 km. There's also a free wildlife walk with kangaroos and birds.

Mount Gambier is halfway between Adelaide and Melbourne on the Princes Highway, making it a perfect overnight stop.