Cocklebiddy Cave is the most famous cave dive in Australia and one of the classic long cave penetrations of the world. The CDAA site description summarises it: an entrance chamber over 300 m long opens onto a 180 m lake, beyond which runs a single, straight tunnel more than six kilometres long, more than 90% underwater, following a fault due north. The first sump runs about 1 km to Rockfall Chamber, a large air chamber blocked by a rockpile about 25 m high; the second sump runs 2.5 km to Toad Hall; and the third about 1.8 km further.

Exploration began in the early 1970s. South Australian divers Hugh Morrison, Keith Dekkers and Phil Prust pushed 500 m in 1974 and reached the big chamber at the 1,000 m mark in 1976; the tunnel floor lies at about 13 m with the roof at 0-10 m. In 1983 Cocklebiddy became the world's longest known underwater cave after French and Australian expeditions pushed beyond Toad Hall, and later expeditions (including 1995 and 1997) extended the surveyed length beyond 6 km.

The water is gin-clear, often brackish, and the tunnel is enormous – divers describe elemental darkness in passages so big that lights fail to reach the walls, which can induce a form of agoraphobia. The dry chambers are huge boulder piles that must be crossed carrying equipment.

Access is tightly controlled. CDAA rates the cave Advanced Cave, and divers must apply in writing to DBCA at Esperance at least four weeks ahead with CDAA indemnity forms. It is extraordinarily remote – more than 1,100 km from Perth – with no rescue capability nearby, so every trip is a self-contained expedition. For qualified cave divers, the first sump alone is an unforgettable dive.

The Nullarbor caves lie along the Eyre Highway between Norseman and the SA border, roughly 11-14 hours' drive from Perth. Cocklebiddy Roadhouse (fuel, meals, water, showers) is the base for the central caves; Norseman, 435 km west, is the nearest regional centre. Access to the cave entrances is by unsealed station/bush tracks (4WD recommended), and all cylinders, compressors, gas, water and rope/hauling gear must be carried in.

Cocklebiddy Cave is a short drive by track from Cocklebiddy Roadhouse on the Eyre Highway. From the doline, divers carry equipment down through a 300 m entrance chamber to the 180 m first lake. The cave is on land managed by DBCA; permission is mandatory and the entrance may be gated or signed closed to the public.

Surface weather matters mainly for access and safety: summer temperatures can exceed 45°C, making gear hauls dangerous, while winter nights are cold. Heavy rain is rare but can flood entrances and destabilise rockpiles (the 1988 Pannikin Plains collapse followed a severe storm). Underwater conditions are stable year-round: groundwater at about 19°C (24°C at Tommy Grahams), often brackish to saline with haloclines, and visibility commonly 20-50 m or more unless disturbed.

Autumn to spring (April-October) is the usual expedition season. Cocklebiddy's long, shallow tunnel is stable, but the large dry rockpile chambers between sumps are hazardous to cross with heavy equipment.

Typical recreational (Advanced Cave) dives explore the first sump from the entrance lake, following the permanent/laid line north along the straight tunnel at 10-15 m on thirds. Deeper penetration to Rockfall Chamber (about 1 km) and Toad Hall (a further 2.5 km) is expedition diving with DPVs, stage cylinders or rebreathers and support teams.

CDAA Advanced Cave rating required. Apply to DBCA Esperance at least four weeks in advance. Thirds rule minimum; long penetrations use DPVs, stages or CCR.

Expedition planning includes compressors/boosters, oxygen, satellite communications and a surface support team.

No operator. Expedition-style trips by CDAA Advanced Cave-rated divers only, with DBCA permission.