Mount Gambier is the home of The Blue Lake which is a flooded, volcanic crater which is on average 80 m deep with a rumoured deep section of more than 200 metres (see photo). The surrounding countryside is thick limestone pitted with sinkholes and cave entrances. The shallow water table means several dozen of them are diveable. There are even caves running underneath the town itself where you can cavedive (ie Engelbrechts Cave).
The sinkholes are generally deep (40-70m), dark and cold (14-16C). Some have crystal clarity while others are like soup that changes consistency between seasons. The most amazing sinkhole has to be The Shaft which was dived by Sheck Exley in the late 90’s to in excess of 80m. Efforts in The Shaft during recent years on trimix have discovered massive deep tunnels at -125m but they have been terminated. Tank Cave is the prize of cave divers in the region and has in excess of 7km of surveyed phreatic maze passage.