Broken Bottom is a scattered natural reef about 2 km north-west of Glenelg, close to Leatherjacket Alley. It is part of the same ancient shoreline that forms Adelaide's inshore reefs, but here the rock has broken up into a series of piles spread over a wide area of sand at 10-11 m. The Sea Wolves describe the colours of the sponges and flora as exceptional, and this is the site's real strength.

The rock piles are covered with sponges in reds, oranges and yellows, sea tulips, ascidians and algae, and on a clear day the shallow light makes them glow. Between the piles the sand holds razor shells, and spider crabs are common. Rays rest on the sand.

Fish populations vary from visit to visit: some days there are schools of old wives, blue devils, leatherjackets and wrasse; on others the reef is quieter. Because the piles are separate, the dive involves moving from one to the next, and navigation needs care. Carry a compass, note the bearing and distance to the anchor, and keep the dive compact if visibility is low.

A surface marker buoy is sensible, as boats heading north from Glenelg and West Beach pass close by and there is no mooring to mark the site. Broken Bottom suits new divers and photographers wanting a shallow, colourful reef, and it pairs naturally with Leatherjacket Alley for two shallow dives in a morning. Like all inshore Adelaide sites, it is at its best after several days of offshore easterly winds, and quickly becomes murky after westerlies, onshore swell or heavy rain.

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 5-10 minutes north-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.

Anchor near the GPS mark and swim a slow expanding circuit between rock piles at about 10 m, using a compass to keep track of the anchor. Look for razor shells and spider crabs on the sand between piles.

No regular charter; club and private boats from West Beach or Glenelg.