CD's Bommie is an offshore pinnacle in the Althorpe Islands area described by the Marine Life Society of South Australia as a big bommie starting at about 32 m and rising to within 12 m of the surface, popular with local charter vessels, with very diverse marine life in huge quantities. Isolated pinnacles like this concentrate life: current sweeps nutrients past the rock, and the flanks are covered in gorgonian fans, sponges and soft corals, while fish school around the structure. The Society lists western blue groper, southern blue devils, bullseyes, snapper and boarfish among the residents.

The depth range is the defining feature – from a comfortable 12 m top to more than 30 m at the base – which means divers can tailor their profile but must watch their depth and gas. Strong currents are likely in open water near Althorpe, so slack-water timing and a well-managed shot line are important. No position is published and there is no regular operator today, so the site is only accessible to those with local knowledge.

It is recorded here with low data confidence as part of the Althorpe dive area. Divers who reach it should treat it as a deep open-water dive: carry an SMB and a reel, stay on the shot or bommie at all times, and plan conservative profiles, remembering that the nearest recompression facility is in Adelaide, several hours away by road or aeromedical retrieval. On a calm, clear day the combination of visibility and fish life makes it one of the standout dives of the lower Spencer Gulf.

From Adelaide drive about 300 km (3.5 hours) via Port Wakefield, Minlaton and Warooka to Marion Bay; Dhilba Guuranda-Innes National Park begins just west of the township (vehicle entry fee, pay online or at the Stenhouse Bay visitor centre). Boat access from Marion Bay – the bommie is described as some distance from Althorpe Island, about 20 km and 40-50 minutes from the ramp. No position is published; it is known to local charter skippers.

Open-water crossing needs settled weather.

The southern tip of Yorke Peninsula faces the open Southern Ocean and Investigator Strait, so south-westerly and southerly swell is the controlling factor: most exposed sites need swell under about 1.5 m and are best after several days of northerly or north-easterly winds, which blow offshore along this coast. Winter cold fronts bring big south-westerly swell and strong winds and can close the area for days. Summer afternoon south-easterly sea breezes make boat runs rough, so launch early.

Visibility is typically 10-20 m in settled conditions but drops sharply with swell-driven surge. Offshore bommies around Althorpe are exposed to strong currents; dive at slack water.

Descend the shot to the bommie top at 12 m, spiral down the flanks to your planned depth (not beyond 30 m on air), and work back up around the bommie, ascending the shot for a safety stop.

Depth range suits nitrox for the top section; anything beyond 30 m should be planned conservatively given distance to the chamber.

Described as popular with local charter vessels (historically); currently no regular operator – private boats and club expeditions with local knowledge.