The Stanvac Mooring Blocks appear in the Sea Wolves Dive Club's list of Adelaide metro sites with a GPS position and a depth of 25 m, but without a written description. They are presumed to be the heavy concrete or steel blocks that anchored tanker moorings serving the former Port Stanvac oil refinery, which operated from the 1960s until 2003 and whose jetty was demolished in 2016. By analogy with other mooring-block sites around Australia, large blocks and associated chain on an otherwise sandy gulf seabed at 25 m typically become heavily encrusted with sponges and ascidians and attract schools of bullseyes, old wives, leatherjackets and morwong, with wobbegongs and cuttlefish sheltering under and around them.

This is a reasonable expectation rather than a documented description. The depth puts the site in Advanced territory, with no-decompression limits on air of around 20-25 minutes depending on the profile. Being offshore in the gulf, it can also be affected by tidal current.

A shot line, a boat attendant and a surface marker are essential. This is a site for experienced local divers keen to explore something off the usual list. Because information is so limited, visitors should check current Port Stanvac exclusion zone arrangements, get the latest mark from the Sea Wolves, and consider reporting what they find to the Scuba Divers Federation or Marine Life Society of SA.

It may be combined with the Stanvac Dump or Barges on the same outing, making a worthwhile half-day from O'Sullivan Beach when the gulf is flat.

Boat access only. The usual launch is the O'Sullivan Beach boat ramp (off Galloway Road, O'Sullivan Beach; multi-lane ramp with breakwater, large trailer park and toilets), about 30 minutes' drive south of the Adelaide CBD. From the ramp run north-west along the coast to the GPS mark.

There are no public moorings: anchor in sand clear of the structure and keep a boat attendant on board with a dive flag, as fishing boats work these sites heavily. Charters from Adelaide Scuba and club boats sometimes use Glenelg or West Beach instead.

Like the rest of the metro coast, these sites face west into Gulf St Vincent and are best in offshore easterly winds, which are most common on summer and autumn mornings and on clear winter days. Westerly and south-westerly winds build chop quickly and make anchoring and recovery uncomfortable; the summer south-westerly sea breeze typically arrives late morning to early afternoon. Tidal streams in the gulf are modest but noticeable offshore on spring tides; neap 'dodge tide' days, when the water barely moves, give the easiest diving.

Visibility is usually better than at Glenelg because there is less river and stormwater influence, but drops after strong westerlies. Being further offshore, the deeper sites need a genuinely calm forecast.

Drop a shot on the GPS mark and descend to the blocks at about 25 m. Explore the blocks and any chain in a compact circuit, watching no-decompression time, and ascend on the shot.

EANx 30-32 useful.

No regular operator – club (Sea Wolves) and private boats from O'Sullivan Beach only.