One Tree Sinkhole

One Tree Sinkhole, Mount Gambier

By ScubaDownUnder Team · 2026-09-04

In the limestone country around Mount Gambier, the ground is full of water. Rain that fell on the Limestone Coast filters down through the rock and gathers in flooded caves and sinkholes of astonishing clarity, and One Tree is among them. From the surface it is an unremarkable hole in a paddock; below, it opens into a funnel of pale limestone falling away into water so clear it seems to vanish, down to a great rock pile at thirty metres and passages that drop beyond fifty. It is not a dive for everyone. One Tree is cave diving, reserved for trained, certified cave divers, and one of the serious sinkholes that have made this corner of South Australia a place of pilgrimage for them.

Mount Gambier sits on the Limestone Coast, a karst landscape riddled with sinkholes and flooded caves fed by one of the cleanest groundwater systems on earth. Diving here is managed by the Cave Divers Association of Australia, which grades each site by difficulty and controls access through a permit and certification system born of hard experience, after fatal accidents in the 1970s led the community to professionalise cave diving in Australia. One Tree is one of these permitted sites, rated for qualified cave divers, on private land where access depends on both a permit and the landowner. It is a place that demands respect before a diver ever gets wet.

One Tree is a funnel-style sinkhole. The entrance narrows into a cone of limestone that widens as it descends, and the collapse of the original roof has left a great pile of rock and rubble at around thirty metres, the floor of the open chamber. From that rock pile, passages lead on and down past fifty metres, well beyond the limits of safe diving and into territory only the most qualified and equipped explorers approach. For most cave divers the experience is the descent through the clear water of the chamber, the play of light from the surface, and the sheer scale of the limestone, rather than the deep passages below. The water is fresh, still and cold, and the visibility, in the absence of silt disturbance, is extraordinary.

This is not a dive about marine life. The sinkhole's freshwater holds little beyond filamentous algae and bacterial films on the rock, a scatter of freshwater crustaceans in the silt, and the occasional eel. What draws divers is the geology and the water itself: the pale architecture of the limestone, the shafts of light reaching down from the single opening, and a clarity that makes divers feel suspended in air rather than water. It is a dive for the senses and the skills, not for the fish.

The conditions in a Mount Gambier sinkhole are stable and unforgiving in equal measure. The freshwater sits at a near-constant temperature, around 15 to 17 degrees year-round, cold enough for a drysuit on a long dive, and there is no current and no swell. Visibility is exceptional when the silt is left undisturbed, often well beyond twenty or thirty metres, but a careless fin stroke can reduce it to nothing in seconds, which is one of the central skills of cave diving. The depth is the other defining factor: the rock pile at thirty metres is deep, and the passages below it are deeper still, so gas planning, narcosis and the discipline of an overhead environment all apply from the moment a diver leaves the surface. There is no direct ascent to the open sky from the bottom.

One Tree belongs to a constellation of Mount Gambier sinkholes and caves, alongside better-known sites like [Kilsby Sinkhole](https://www.scubadownunder.com/dive-sites/kilsby-sinkhole), [Piccaninnie Ponds](https://www.scubadownunder.com/dive-sites/piccaninnie-ponds) and [Ewens Ponds](https://www.scubadownunder.com/dive-sites/ewens-ponds), and serious cave divers often work through several on a single trip. It is firmly at the technical end: this is not a site to be dived on an open-water ticket or out of curiosity, but a graded cave dive for those who have done the training, hold the right qualification and carry the redundancy that cave diving demands. For divers who have earned their way into it, it is one of the quiet wonders of the Australian underground.

One Tree distils what makes the Mount Gambier sinkholes extraordinary: impossibly clear freshwater, the pale geometry of drowned limestone, and a stillness found nowhere in the sea. It is also a reminder that some of the best diving in Australia is also the most demanding, open only to those who treat the overhead environment with the seriousness it requires. Dived properly, by the qualified, it is unforgettable.

## Site Access and Logistics

One Tree is a freshwater sinkhole near Mount Gambier, in South Australia's Limestone Coast, about four and a half hours from Adelaide and a similar distance from Melbourne. It is a land-based dive entered from the sinkhole itself, on private property, and there are no commercial dive facilities at the site. Mount Gambier town, a short drive away, provides air fills, accommodation and the base for most trips.

This is a cave dive, not a recreational one. Access requires a permit and membership of the Cave Divers Association of Australia, and the site is graded for divers holding the appropriate CDAA cave qualifications, with the deeper passages beyond the reach of all but the most advanced and properly equipped. Cave training, redundant gas and lights, guidelines and strict buoyancy and silt discipline are mandatory, not optional. Open-water divers cannot dive One Tree. The water is cold and fresh, so a drysuit suits longer dives.

Access, grading and permits are coordinated through the [Cave Divers Association of Australia](https://www.cavedivers.com.au), and Mount Gambier operators support trips with fills and gear.

## Sources

- Scuba.com, dive the craters and sinkholes of Mount Gambier: [https://www.scuba.com/blog/dive-craters-sinkholes-mount-gambier/](https://www.scuba.com/blog/dive-craters-sinkholes-mount-gambier/) - Visit Mount Gambier, sinkholes: [https://visitmountgambier.com.au/travel-type/sinkholes/](https://visitmountgambier.com.au/travel-type/sinkholes/) - We Are Explorers, sinkhole caves and underwater rainforests in Mount Gambier: [https://weareexplorers.co/sinkhole-caves-underwater-rainforests-mount-gambier/](https://weareexplorers.co/sinkhole-caves-underwater-rainforests-mount-gambier/) - Curious Campers, Mount Gambier sinkholes and caves: [https://curiouscampers.com.au/mount-gambier-sinkholes-and-caves/](https://curiouscampers.com.au/mount-gambier-sinkholes-and-caves/) - Cave Divers Association of Australia: [https://www.cavedivers.com.au](https://www.cavedivers.com.au)