One Tree is a large, deep farm sinkhole in the Mount Schank field, known mostly to CDAA members. The association's listing is short: large deep sinkhole, visibility usually good below 20 m, thermoclines, a harvester and some rubbish. Like many of the region's farm holes it was once used as a convenient place to dump unwanted machinery, and the old harvester on the bottom has become a landmark.

The dive profile is typical of the Mount Schank sinkholes: the upper layer is often greener and murkier, then one or more thermoclines mark the change to colder, clearer water below 20 m, where the walls and the depth of the hole open up. The open daylight zone makes it a Basic Cave site, and the depth means that narcosis and gas planning deserve attention even without an overhead.

What distinguishes One Tree is its old-fashioned access. There is no online booking – divers call at the house, show a CDAA card, sign the logbook and collect the keys, and the rules are explicit that if nobody is home or you do not have your card, you do not dive. That makes it a site to plan with a phone call and some flexibility. Exact depth and dimensions are not published. It suits CDAA Basic Cave divers looking for a quieter, deep sinkhole with a bit of agricultural archaeology on the bottom. Because access depends on the landowner being at home, many divers treat it as a back-up option on a Mount Gambier trip, calling ahead in the morning and diving if the family is around. Visiting divers should leave gates as found, keep clear of stock, and thank the owners – the future of farm access across the region rests on that goodwill.

One Tree is on LG McKinnon & Sons' property in the Mount Schank area, about 15-20 minutes south of Mount Gambier. There is no online booking: call past the farmhouse to collect the keys before diving, show your current CDAA membership card and sign the logbook. If no-one is home there is no diving, and without your card there is no diving.

Drive to the sinkhole on the farm track as instructed, park clear of stock and gates, and carry gear to the edge.

Unaffected by swell or tide – the limiting factors are access and surface conditions on private farmland. Summer brings heat, snakes and (at some sites) aggressive bees; Total Fire Ban days can close forestry and some private sites and ban compressors; winter rain turns paddock tracks muddy. Shallow water in sinkholes grows algae in summer, reducing visibility in the top few metres; the water below the thermocline is usually clearer and a constant ~15°C.

Descend through the shallower, murkier water and thermoclines to the clear layer below 20 m, circuit the walls within Basic Cave depth limits and the daylight zone, and avoid the harvester and rubbish on the bottom. Ascend along the wall for a stop in the shallows.

CDAA grade: Basic Cave (5L7), owner LG McKinnon & Sons. Keys collected in person; CDAA card and logbook sign-in mandatory. Stay in daylight zone and within 40 m Basic Cave limits.

No operator – CDAA Basic Cave members arrange access in person with the landowner.