The Three Sisters is one of the most physically demanding cave dives in the Mount Gambier region, and one of the oldest documented – the cave has been known since at least 1933. It originally had three roof windows, hence the name, but today there is a single gated entrance a few metres off Glencoe Mile Hill Road near Tantanoola. Entry is through a 1 m concrete pipe followed by an 18 m vertical drop, climbed on a wire rope ladder with every person clipped to a belay.

At the bottom is a large talus mound – a cone of collapse debris, with glass fragments and other rubbish that fell through the windows over the decades – leading down into a dome chamber about 30 m in diameter. The main chamber reaches about 35 m. Beyond it the cave's notorious 'flattener', a restriction only 30-40 cm high that runs for several metres, is the reason the site is sidemount-only and graded Advanced Cave.

Rules are extensive and reflect both the vertical entry and the roadside location: helmets mandatory inside the cave, one responsible adult topside, a maximum of four divers in the water, one vehicle on the hard pan, hi-vis vests and cones, and the key collected from the Glencoe Post Office and Bistro. Wattle Range Council owns the site. The dive rewards fit, experienced Advanced Cave sidemount divers with a big, dramatic chamber and a genuinely committing restriction; for everyone else, the ladder alone should give pause. Allow a full morning for rigging and traffic management.

Collect the key from the Glencoe Post Office and Bistro, 193 Glencoe Road (9 am-5 pm daily), about 20 km north-west of Mount Gambier. The gated entrance is a few metres west of the sealed Glencoe Mile Hill Road near Tantanoola. Only one vehicle may park on the hard pan near the entrance; others park on the fire track behind the cave, never across Mile Hill Road.

Wear high-visibility vests and set out traffic cones. Everyone must be clipped to a belay before approaching the entrance; divers climb down the 18 m wire rope ladder through the pipe. Contact the Site Access Officer a week ahead if unable to climb.

Unaffected by swell or tide. Access and surface conditions on private farmland are the limiting factors: Total Fire Ban days may restrict access or compressors, summer heat makes vertical exits hard work, snakes are active in warm weather and winter rain turns farm tracks muddy. Underwater conditions are stable, with groundwater at a constant ~15°C.

The entrance is beside a sealed road, so traffic management applies in all weather; rain makes the ladder and talus slippery.

Descend the ladder, kit up at the base of the talus mound, and descend into the 30 m dome chamber on a guideline. Pass the 'flattener' restriction (30-40 cm high, several metres long) only in sidemount and within Advanced Cave protocols, explore to a maximum of about 35 m, and turn on thirds. Helmets stay on inside the cave.

CDAA grade: Advanced Cave (5L13), owner Wattle Range Council; sidemount only. Helmets mandatory; max 4 divers in water; one adult topside; belay for all near entrance. Continuous guideline, thirds or stricter, three lights; nitrox 28-30% for 35 m.

No operator – CDAA Advanced Cave sidemount divers; max four divers in the water; Site Access Officer contact via CDAA.