Mount Gambier

Mount Gambier sits on a volcanic complex that last erupted around five thousand years ago, and the city's water supply comes from the Blue Lake in one of its craters. Each year around November the lake changes from a dull steel grey to an intense cobalt blue, holds that colour through summer, and reverts in March, a shift linked to calcium carbonate crystallising in the warming water and scattering light, though the details are still studied. Beneath the city, limestone caves open directly into the streets: the Cave Garden is a sinkhole in the town centre with a waterfall, and Umpherston Sinkhole has been planted as a terraced garden reached by steps.

What you actually see

• The Blue Lake - the crater lake that turns cobalt each summer.
• Umpherston Sinkhole - a collapsed cave planted as a terraced garden.
• The Cave Garden - a sinkhole with a waterfall in the middle of the city.
• The volcanic craters - Valley Lake and the crater rim walks above town.

When to go

December to March for the blue colour, which fades by late autumn; from April to October the lake is grey. Umpherston Sinkhole is lit at night and possums come out to feed. Winter on the Limestone Coast is cold and wet.

Honest expectations

Outside summer the Blue Lake is genuinely grey and disappoints visitors who arrive expecting the photographs. The lake is a drinking water supply, so swimming and boating are prohibited. The city itself is a working regional centre.

Nearby

Piccaninnie Ponds and the coast lie south; Coonawarra and Penola are north.