Mount Gambier's big-ticket item is the luminous, 75m-deep lake, which turns an insane hue of blue during summer. Perplexed scientists think it has to do with calcite crystals suspended in the water, which form at a faster rate during the warmer months. Consequently, if you visit between April and November, the lake will look much like any other − a steely grey. Acquifer Tours runs hourly tours, taking you down near the lake shore in a glass-panelled lift.
