Hells Hole is a big, raw forest sinkhole – the CDAA summary is 'large sinkhole dropping 30 metres to the water; visibility varies, much tree debris'. It lies on ForestrySA land in the pine plantations around Mount Gambier, one of six wet cave sites the forestry agency manages for CDAA divers, and it is as much a rope-work exercise as a dive.

The defining feature is the drop: the water surface lies about 30 m below the rim, so teams need vertical access gear, a plan for lowering cylinders, and the fitness to climb back out afterwards. Once in the water the sinkhole is open to daylight, with walls falling away into water whose clarity changes with season and recent rain. Decades of fallen trees and branches lie in the water and on the floor, creating a jumble of timber that adds atmosphere but also entanglement and silt risk. It feels remote and wild compared with the pontoons and stairways of Kilsby's or Little Blue.

It is a Basic Cave site, meaning divers stay in the daylight zone with no penetration beyond it. Access is through the ForestrySA online booking system with a key from the Mount Gambier Visitor Information Centre, and the forest closes on Total Fire Ban days. It suits experienced, self-sufficient CDAA Basic Cave teams who enjoy vertical access and a less-visited, atmospheric site. Teams usually allow half a day, with at least one person staying topside to manage ropes and cylinders, and the site rewards those who treat the rigging as seriously as the dive itself.

Book online through the ForestrySA caving portal (select Hells Hole and the date, enter diver details); ForestrySA recreation staff can help on 08 8391 8800 in office hours. Collect the key from the Mount Gambier Visitor Information Centre (Lady Nelson), then drive into the forest south-west of Mount Gambier on forestry tracks as per the booking instructions. Park at the rim; the water lies about 30 m below, so teams rig ropes/ladders and lower cylinders separately.

Forestry is closed on Total Fire Ban days.

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.

The 30 m climb out is hard work in summer heat; start early.

After rigging and lowering gear, descend the wall and explore the open sinkhole within the daylight zone, avoiding the tangle of fallen trees. Stay off the floor, turn with ample gas for a relaxed exit, and allow plenty of time and energy for the climb.

CDAA grade: Basic Cave (5L40), owner ForestrySA – book via forestrysa.com.au/caving; key from Mount Gambier Visitor Information Centre. Vertical rigging (ropes, ladder, harness, belay) required. Remain in daylight zone; carry cutting tool for debris.

No commercial operator – CDAA Basic Cave members via ForestrySA booking.