Bum Rock is the Glennie Group's landmark: a massive smooth granite boulder on the western side of Dannevig Island split by a large central crack that, as Scuba Doctor puts it, looks just like its name. The crack runs from above the waterline about 10 m down into the rock, and the boulder stands on a boulder-covered bottom at 30 m or more, so the dive is a descent through or beside the cleft to the base and a circuit of the giant block and its neighbours. It is a wall dive in effect – the sides of the boulder are sheer for 20 m – and, being on the exposed western face of the island in the path of every Bass Strait swell, it carries growth to match: a band of bull kelp in the surge zone, then sponge, zoanthid, gorgonian and sea-whip cover thickening with depth, jewel anemones and bryozoans in the crack and under the overhangs, and crayfish deep in the base.

Fish schools – butterfly perch, sweep, bullseyes – hang in the eddies on the lee side of the rock, boarfish and morwong work the ledges, and seals from the Glennie haul-outs often follow divers around the boulder. The hazard is surge, which Scuba Doctor describes as very strong; the crack concentrates it and the shallow part of the dive can be a washing machine on days when the rest of the island is diveable. Bum Rock is therefore a rare-day dive even by Prom standards, attempted when a club or private boat at Tidal River finds an easterly wind and a flat western swell, and it is prized as much for the photograph of the boulder from the boat as for the dive.

For an Advanced diver in the right conditions it is a memorable, almost sculptural piece of granite topography with the clear water and colour of the outer Glennies.

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. Bum Rock is on the exposed western side of Dannevig Island, about 8 km from the Tidal River beach launch; it can only be dived when the western swell is small, from a live boat or an anchor set in sand well clear of the boulder.

On the western, Bass Strait-facing side of the island and subject to very strong surge, in Scuba Doctor's words; diveable only with a swell well under 1 m and light winds from the east. The crack funnels surge and is dangerous in any sea. Rare autumn calms only.

The eastern Prom coast is reached only after a long boat run from Port Welshpool, so the whole day's forecast must be settled: swell under 1–1.5 m, light winds, and no change due. Easterly and south-easterly swell is the main problem on this coast; a westerly wind flattens it but makes Corner Inlet's entrance choppy on the ebb. Visibility is typically 10–25 m and best in autumn after several calm days.

From a live boat, descend beside the boulder and drop through or alongside the central crack to the boulder-covered bottom at 25–30 m, then circle the base of the rock and the surrounding boulders, keeping out of the crack if there is any surge. Turn at half gas, ascend the outside of the boulder to the kelp at 5–10 m and finish under an SMB for pick-up.

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.