A touch of blue

South Australia's Mount Gambier, located about halfway between Melbourne and Adelaide, is built on a warren of limestone caves and sinkholes. Its gleaming, white limestone is quarried and has been used for many of the city's imposing buildings.

Mount Gambier's best-known natural icon is the remarkable Blue Lake in one of the four volcanic craters around which the city is built. The lake's vivid, sombre, inky blue colour remains during winter. Early in November, with the onset of summer, the colour mysteriously changes, almost overnight, to an intense turquoise blue. The most popular current scientific theory holds that microscopic calcite particles in the warmer summer water absorb every colour in the spectrum except blue. These particles dissolve when the water cools.

Whatever the explanation, you can view the phenomenon from a newly constructed pedestrian walkway around the Blue Lake crater rim. The water is safe for drinking - it forms the city's water supply. A trip run by Aquifer Tours takes visitors in a glass-panelled lift down the original limestone bore-well, through a tunnel and onto a pontoon at the lake surface.

Not far away, Leg of Mutton Lake (dry since 1859) offers lovely walking trails. Or try snorkelling in Ewen Ponds, where the water is so clear that plants grow to a depth of six metres. In nearby Piccanninie Ponds, you can snorkel across the top of The Chasm and often see down to a depth of 40 metres.

And when you need to recharge the batteries, head to the historic port of Robe to feast on local food and wine, check out the eclectic mix of shops and wander among the restored buildings.