The Glenelg Blocks are a line of hollow concrete blocks running parallel to the beach about 500 m off Glenelg, the remains of an old ship mooring system associated with the early Glenelg jetty era. They are spaced over about 200 m in only 5-6 m of water, and at low tide their tops can be seen breaking the surface. What makes the Blocks worth a dive is the life that has made them home.

Wobbegong sharks are almost always found resting in and around the blocks, according to the Sea Wolves, and the concrete is covered with algae, sponges and small invertebrates. The sand between is classic muck-diving ground: the Federation describes it as an interesting second dive and an excellent muck dive. At night the site changes character, with dumpling squid, octopus, southern calamari, sea stars and urchins emerging.

The southern pillars have small swim-throughs. Being so shallow and so close to Glenelg, the Blocks are a common choice for second dives after the Dredge, for night dives, for training and for divers wanting a long, relaxed hour looking for small things. The flip side is visibility: the site is readily stirred up after strong winds and affected by runoff from the Patawalonga.

Pick a calm morning after easterlies. Boats are the practical way to visit, but experienced swimmers can reach the Blocks from the beach on a calm day. Either way, fly a flag or use a surface marker buoy, because this is a busy stretch of water with jet skis, fishing boats and boats heading in and out of Holdfast Shores marina.

Most divers visit by boat: a two-minute run from the Glenelg (Holdfast Shores) or West Beach ramps, anchoring in sand beside the blocks, which are marked by navigation aids. The blocks can also be reached from shore by strong swimmers: park along the Glenelg foreshore (paid parking around Moseley Square and Holdfast Shores), enter from the beach north of the jetty and fin about 500 m offshore on the surface. This is a long swim in full gear and should only be attempted on calm, clear days, with a surface marker because boats use the area heavily.

Shallow and close to the beach, the Blocks are ruined by any westerly or south-westerly wind or swell, which stirs the sand and brings in suspended sediment. Dive after several days of easterly (offshore) winds, early in the day before the summer sea breeze. Low tide exposes the tops of the blocks, which makes them easy to find but reduces depth over them to a metre or so; mid to high tide is more comfortable.

Visibility falls sharply after rain because of outflow from the Patawalonga and stormwater drains.

Anchor or drop beside the southern blocks, then swim north along the row at 4-6 m, checking under and between each block for wobbegongs and small invertebrates. The southern pillars have small swim-throughs. Turn at half gas and swim back along the seaward side.

Used by Adelaide boat operators and clubs as a shallow second dive or night dive; no scheduled trips. Private boats from Glenelg or West Beach.