Olwolgin Cave is the modern headline of Nullarbor cave diving. Exploration began in 2001 under Paul Hosie, and major underwater passages were discovered in 2011. By January 2015 more than 11,000 m of passage had been surveyed in the downstream sector, making it Australia's longest known underwater cave system. The passages are shallow – about 10 m beneath the surface – and include named sections such as Grand Central (130 m long, 30 m wide), Babylon Lake with its 'hanging gardens', and Blood Moon Passage. Divers report white-coated walls, organic growth mats, toxic atmospheres in air spaces and an ancient dingo skeleton on a passage floor.

The entry is comparatively easy – a 1.3 km bush walk to a small daylight pool – but the diving is extreme: exploration uses sidemount rebreathers with bailout, with dives lasting more than four hours away from base. The CDAA rates Olwolgin Advanced Cave. It offers immense scope for scientific research, and exploration is far from complete.

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.

Olwolgin lies on the Roe Plain below the escarpment. From the track head, a 1.3 km walk through bush leads to the small entrance pool, reached by a relatively easy scramble. All equipment must be carried in.

Permission is required through CDAA/land-manager procedures.

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. Olwolgin is shallow (about 10 m below the surface) and parts have toxic atmospheres in air spaces.

Exploration diving on sidemount rebreathers with bailout; dives exceed four hours away from base, requiring constant re-evaluation of gas and equipment.

CDAA Advanced Cave. Sidemount CCR with bailout; multi-hour dives; exploration protocols.

No operator – exploration teams (typically sidemount CCR divers) only.