Skull Rock, officially Cleft Island, is the great granite dome that stands alone in Bass Strait west of the Anser Group, hollowed on its northern face by a sea cave large enough, as the Prom cruise operators like to say, to hold the Melbourne Cricket Ground. Seen from the sea the eroded hollow and the two dark overhangs give it the look of a skull, and it has become the symbol of the Prom's wild south. Underwater it is, in Scuba Doctor's words, awesome diving: the rock rises 113 m above the water and the granite continues 65 plus metres below it as near-vertical walls, with underwater caves, and it is rated Deep/Technical for that reason.

The dive is a wall dive in the purest sense. Below a fringe of bull kelp in the surge zone the granite drops sheer or in giant steps, and because there is no shelter from swell or current every surface below 15 m is covered in filter feeders – sponges in every colour, yellow zoanthids, gorgonian fans, sea whips, jewel anemones and bryozoans – in a density that only the most exposed sites in Victoria achieve. Fish hang off the wall in clouds: butterfly perch, barber perch, sweep and bullseyes close in, with kingfish, salmon and the occasional sunfish in the blue.

Australian fur seals from Kanowna and the Anser Islands haul out on the lower ledges and, the listing notes, frequently interact with divers. Visibility is often 20–30 m, the clearest water on the Victorian coast. The caves at depth are large granite overhangs and clefts rather than true cave systems, but they and the seals give the shallower part of the dive its character.

Everything about Skull Rock is demanding except the diving itself: the site is remote, the walls invite depth, the current runs, the seas are often wild, and it is reached only by a well-found private or club boat on a rare calm day. Parks Victoria's Wilsons Promontory Cruises run sightseeing boats to the cave from Port Welshpool, which is how most people see it. For divers with the boat, the weather and the experience, it is the finest wall dive in Victoria.

Boat only. The nearest launch is the Tidal River beach ramp at the western end of Norman Bay (4WD beach launch, key and deposit from the Parks Victoria office; needs several people and calm surf), from which the western islands are 5–12 km. Larger trailer boats launch at Port Welshpool (sealed ramp, Corner Inlet) and run about 50–60 km around South East Point – 1.5–2 hours – or at Sandy Point (Waratah Bay, beach ramp) and run 35–45 km south-east.

Sites are exposed to Bass Strait and require settled weather, a seaworthy boat, radio, and self-sufficiency: there is no rescue or refuelling closer than Port Welshpool. Skull Rock is the most distant of the western sites – about 14 km from Tidal River and 60 km from Port Welshpool – and is usually only attempted on a forecast of light winds and swell under 1 m for the whole day. Anchor on the lee side in the sand at 20–30 m or keep the boat live; the walls drop away too steeply to anchor close in.

Scuba Doctor's advice is calm seas, no swell and no wind, with light offshore north-easterly to south-easterly winds acceptable, and slack water because of strong currents; the rock sits in open Bass Strait with no shelter from the prevailing south-westerly, and the seas are described as often wild. A long boat run in a trailer boat means the whole day must be settled, not just the morning. Late summer and autumn high-pressure systems give the best chance.

Great white sharks frequent the seal colonies of the Anser Group.

Descend the anchor or shot line on the lee side and follow the granite wall down to a planned maximum (30–40 m for recreational divers; the wall continues past 65 m), then work laterally along the face and into the shallower caves and gutters at 10–20 m where the seals play. Turn on a third of gas, return to the line and make a long safety stop on it or under an SMB; the boat should be live and watching, as current can carry a diver off the rock.

Rated Deep/Technical by Scuba Doctor; wall continues beyond 65 m and suits trimix/rebreather divers; recreational divers should plan a hard floor at 30–40 m. EAN32 for the 20–30 m band.

No dive charter operates at Wilsons Promontory. Sites are dived from club and private trailer boats – Dive La Trobe, VSAG, Melbourne University Underwater Club and Dive Victoria Group run occasional camping-and-diving trips – launching at Tidal River (beach launch, permit/key from Parks Victoria), Port Welshpool or Sandy Point. Wilsons Promontory Cruises (Port Welshpool) runs sightseeing cruises to Skull Rock and the islands but no diving.