The Blue Lake is Mount Gambier's emblem and the source of the city's drinking water, and it is included here because divers constantly ask about it. It is a volcanic maar crater lake: the crater rim measures about 1,200 x 824 m and the lake about 1,087 x 657 m, with a basically flat floor averaging around 72 m deep and a deepest point of 77 m recorded in 1967. The water is high-quality groundwater from the limestone aquifer.

Its fame comes from colour. Each November the lake turns a vivid turquoise-cobalt, gradually returning to a duller blue or steel grey from late February to March. The likely mechanism is calcium carbonate precipitating as microcrystallites in warmer months, scattering blue light.

Recreational diving and swimming are not permitted because the lake is a protected public water supply. Diving has happened only as authorised research. In 1985 cave diver Peter Horne led a major exploration that found freshwater sponges and the lake's Stromatolite Field, and he coordinated four more studies up to 1991; another exploration was authorised in 2008 under Dr Richard Harris. The lake is also known for syncarid crustaceans shared only with Fossil Cave.

For visiting divers, the Blue Lake is a surface attraction: walk the rim, take the Aquifer Tour down the old pump shaft, and then go and dive Kilsby's, Ewens or – with the right CDAA grade – the region's caves. It is not a dive site and should not be treated as one. The lake is also a major tourist site, so treat it as a rest-day stop.

The Blue Lake is on the southern edge of Mount Gambier, reached by John Watson Drive from the city centre (about 5 minutes). There are several lookouts and a 3.6 km rim walk with parking at multiple points. The Aquifer Tours operator offers a lift down the old pumping shaft to a glass-fronted viewing point near the water.

There is no public water access and no recreational diving – the lake is the city's water supply.

Not applicable to recreational diving – the lake is closed to all water activity. For sightseeing, the colour change is the event: each November the lake turns deep turquoise/cobalt, fading to a duller blue or steel grey by late February-March and through winter. Clear, sunny middays show the colour best; overcast winter days make it look grey.

Rim roads can be busy on summer weekends.

Not applicable – no diving permitted.

Not a recreational or CDAA site. Research dives only with SA Water/government authority; depth 72-77 m would require trimix and decompression.

No recreational diving. Research dives only under special authority (e.g. Peter Horne-led projects 1985-1991; a 2008 exploration coordinated by Dr Richard Harris).