Mac's Ground is one of a series of low natural reefs off Glenelg that mark an ancient shoreline drowned when sea levels rose at the end of the last ice age. On an otherwise flat seabed of sand and seagrass, these ledges are islands of hard substrate, and fish gather on them in numbers out of proportion to their size. The reef is small: about 150 m long, running east to west, and rising only about 1 m above the surrounding sand in 17 m.

What it lacks in relief it makes up for in texture. The edge is undercut in places into overhangs, and there is a small cave that local guides say is home to squid and cuttlefish. The rock is covered with sponges, ascidians and short algae, and the whole structure is easy to follow from end to end in a single dive.

Fish life is described by the Federation and the Sea Wolves as usually prolific and tame. Blue devils sit in the holes, silver drummer and strongfish cruise the top of the ledge, and bullseyes, old wives and leatherjackets fill the undercuts. Spider crabs are common, and cuttlefish are frequently seen.

Because few boats visit, the fish are relatively unwary, which makes this a good site for photography when the visibility is up. There are no specific hazards listed for Mac's Ground beyond those of any offshore boat dive: finding it needs a GPS mark, and visibility depends heavily on recent weather. It is a good choice for a relaxed, shallow-to-moderate reef dive close to Glenelg, for divers who have done the Dredge and want to see the natural reefs that underpin Adelaide's offshore ecology, and as a quieter alternative on busy weekends.

Boat access only. Adelaide Scuba runs its charters from its shop on the Patawalonga at Glenelg North, with the boat leaving through the Patawalonga/Holdfast Shores entrance; private boats launch at the West Beach boat ramp (Barcoo Road end of the West Beach dunes, large trailer park, fee payable) or the Glenelg (Holdfast Shores) ramp. From Glenelg it is a run of about 10-20 minutes due west in flat water.

Use GPS marks: the sites are low-relief and cannot be seen from the surface, and there are no public moorings, so anchor in sand off the structure and leave a boat attendant with a dive flag up because charter and fishing traffic is steady on weekends.

The metro coast faces west into Gulf St Vincent, so the rule is simple: offshore easterly winds flatten the sea and clear the water, while westerlies and south-westerlies build a short, steep chop and stir up the silty bottom. In summer the south-westerly sea breeze usually arrives by late morning or early afternoon, so boats aim to be back in by lunchtime. Northerlies are workable when light.

The gulf has a mixed, small-range tide with 'dodge tides' (neap periods when the water barely moves) that give the easiest diving; strong spring tides can run noticeably across the offshore wrecks. Visibility drops for several days after strong westerly blows and after heavy rain when stormwater and Patawalonga/Torrens outflow reach the coast.

Drop the anchor on the sand at one end of the reef using GPS. The ledge runs east-west for about 150 m: swim along one side at 16-17 m, checking overhangs and the small cave, turn at the far end and return along the other side to the anchor.

No regular charter; dived by club boats (Underwater Sports, Sea Wolves Dive Club, The Dive Shack) and private boats from West Beach.