Blue Lake, Volcanic crater lake in Mount Gambier, South Australia, Australia

Blue Lake / Warwar is a crater lake in a dormant volcanic maar near Mount Gambier. Its surface measures 1,087 by 657 m, the crater rim 1,200 by 824 m, and the flat bottom averages about 72 m deep. The water lies 17 m below the level of the town's main street.

Estimates for the crater's last eruption conflict: 4,300 years ago, 28,000 years ago, or shortly before 6,000 years ago. If the youngest date holds, it would be the most recent volcanic eruption on the Australian mainland. The Boandik people occupied the area before the colonisation of South Australia.

In February 2022 the City of Mount Gambier assigned dual names to sites of cultural significance to the Boandik people, giving the lake the Bungandidj name Warwar, meaning "crow country". The name sits alongside Blue Lake on signage and maps.

A 3.6 km road and walking track circles the crater rim, with an underpass linking it to Leg of Mutton Lake / Yatton Loo. A rotating lookout northwest of the lake gives a high view over the water and the town of Mount Gambier. A glass lift descends a drained dolomite shaft to an underwater tunnel where the lake and the town's water supply can be seen.

The water shifts to a bright turquoise blue each November and keeps this color until returning to its normal shade in late February. This yearly color change remains a puzzling natural occurrence that intrigues many who visit.