The Shaft is Australian cave diving's most historically important site, and one of the most dramatic sinkhole dives anywhere. From the surface it is almost nothing – a hole about a metre wide in a farm paddock at Allendale East, discovered in 1938 when a horse stumbled into what was then a foot-wide opening, later enlarged for access. Below it lies an enormous flooded chamber roughly 140 m long and 80 m wide, with the water surface about 7 m below the paddock. A central rockpile rises to about 36 m depth, a northwest tunnel reaches 80 m and an eastern tunnel descends to 124 m; CEGSA divers mapped it in detail and surveyed those depths.

The name comes from the column of sunlight that, around midday, drops through the narrow entrance and strikes the rockpile – a spotlight in a void so big that even a 200 W video light cannot illuminate the walls. Visibility is typically excellent, and the effect of floating in black water with a single beam of daylight is unforgettable.

On 28 May 1973 four divers – Stephen and Christine Millott, Gordon Roberts and John Bockerman – went beyond their planned limits without a guideline, became lost and drowned; their bodies were recovered between January and April 1974. The tragedy led directly to the formation of the Cave Divers Association of Australia in September 1973 and to the controlled-access, graded system still in force. The site was closed, successfully mapped in 1984 and later reopened by arrangement with the landowner.

Access today is tightly controlled: CDAA members only, on supervised dives run by approved, insured Dive Supervisors. Basic Cave divers stay around the rockpile top within 40 m; the deeper tunnels are trimix territory. It suits well-drilled CDAA divers comfortable with vertical entries and very large, dark spaces.

The Shaft is on Mr Trevor Ashby's private farmland at Allendale East, about 20 km south of Mount Gambier; directions and gate instructions come with the booking. Park at the paddock as directed, carry gear a short distance to the capped hole, and set up the winch or ladder system. Divers are lowered or climb down roughly 7-8 m through the narrow shaft to the water, and cylinders and equipment are lowered separately on ropes.

The landowner requires all diving to be led by an approved Dive Supervisor carrying their own insurance; book online through the CDAA system well ahead.

Underground and entirely sheltered, but the paddock entry means rain, wind and heat affect the long surface set-up. The famous 'shaft' of light only reaches the rockpile around midday on sunny days, so schedule dives between about 11 am and 1 pm for the effect. Fire-ban days may restrict compressors and vehicles on farmland.

Water is a constant ~15°C.

After the ladder/winch descent, drop through the entry column onto the central rockpile (minimum depth ~36 m), orient on the midday light shaft, and stay within the 40 m Basic Cave limit around the top of the rockpile and in the daylight zone. Technical teams follow fixed lines toward the northwest tunnel (80 m) or east tunnel (124 m) on trimix with planned decompression. Ascend the entry column and complete stops on the line before the climb out.

CDAA grade: Basic Cave (5L158) – dives within 40 m in the daylight zone; deeper tunnels require CDAA Advanced Cave plus trimix and decompression qualifications and landowner approval. Landowner mandates insured CDAA Dive Supervisors lead all diving. Run a continuous guideline from the entry column whenever leaving the daylight zone.

Stage and deco gas lowered separately; rebreathers used on deep dives.

Only with an approved CDAA Dive Supervisor (insured) under the landowner's conditions – booked online through CDAA. Technical operators such as Deep Blue Ventures and Diving Adelaide run supervised trips for CDAA members; no Open Water access.