The Blue Lake is a 70-metre-deep crater lake sitting inside an extinct volcanic maar on the southern edge of the city. What makes it famous — and genuinely baffling — is the colour change: each November the water shifts from grey to a striking cobalt blue, remains that shade all summer, then reverts in March. The scientific explanation involves calcium carbonate precipitation triggered by warming water temperatures, but no model has fully replicated the timing or intensity of the change.
A 3.6 km walking track circles the rim of the crater, giving views down into the water from multiple angles. The lake supplies Mount Gambier's drinking water, so swimming is not permitted, but the walk itself is one of the best easy strolls in the region. The pumping station on the lakeshore runs guided tours during the warmer months.
The Boandik name for the lake is Warwar. It holds deep spiritual significance for the traditional owners and has been a site of ceremony and gathering for thousands of years.