Mount Gambier — Limestone Coast

The Limestone Coast sits on Gambier Limestone, a soft Miocene rock that groundwater has been dissolving for millions of years into caves, cenotes and collapse sinkholes. Mount Gambier itself is a dormant maar volcano whose own crater holds the Blue Lake, a 75 m town water-supply lake that turns from grey to vivid blue each November; it is not diveable, but it sets the scene for a region built on volcanic and karst geology working on each other. The four sites divers actually visit lie in a ring around the city, each a separate flooded doline rather than a connected system.

What makes this area unusual is who runs it. Piccaninnie Ponds and Ewens Ponds are Parks SA conservation parks booked through one-hour permitted time slots; Kilsby Sinkhole and Little Blue Lake sit on private or council land with their own access rules. Independent diving at three of the four requires current membership of the Cave Divers Association of Australia (CDAA) and a site rating, most commonly Basic Cave, which the CDAA also calls sinkhole category on its own permit paperwork. Only Ewens Ponds asks nothing more than an ISO 24801-2 Open Water-equivalent certificate, which is why it is usually the first stop for visiting divers before CDAA membership is sorted.

In the city itself, Engelbrecht Cave is run as a separate tourist cave-diving venue and is often the introduction locals give visiting divers before they head out to the sinkholes proper. Tourism from the whole cluster of craters, caves and sinkholes is worth an estimated 100 million dollars a year to the Mount Gambier economy, and cave divers travel from well outside Australia specifically for it. None of it reads like ordinary scuba diving: there is no reef, no current to speak of at most sites, and the appeal is entirely about looking through impossibly clear fresh water at limestone rather than at fish.