Shark Cave is Rottnest's marquee advanced dive and one of the best grey nurse shark sites in temperate Western Australia. It is a reef lump in the West End Demersal Sanctuary Zone, about a kilometre off the far western tip of the island, with an arc-shaped cave eroded into its side. The top of the reef sits at about 17-18 m and the best section, the cave itself, is at 24-26 m.

The attraction is the grey nurse sharks, a protected species, which rest in and around the cave during the day. Numbers vary: Perth Scuba and Divernet report up to 15 at a time, while Samphire notes one or two is more typical on an average visit. The sharks are slow-moving and inquisitive, and divers sometimes find shed teeth on the sand. Bucket List Diver adds wobbegong sharks and smooth stingrays, and the UWA Underwater Club describes dozens of other fish species on the reef; the sanctuary status means dhufish, groper and other demersals are unusually abundant.

The dive is short because of the depth and demanding because of the location: the West End is exposed to the full Southern Ocean swell and to strong currents where water wraps around the island, and the boat ride is long. For that reason it is dived only a few times a year and is strictly for Advanced divers with good gas management, ideally on nitrox. Divers should stay low, move slowly and never block the cave entrance, which lets the sharks come and go and produces the best encounters.

The West End is the far western tip of Rottnest, around 30 km from Fremantle; Bucket List Diver quotes 1-2 hours by charter boat from the mainland, and private boats need a long run along either coast. Only a handful of days a year are calm enough, so operators schedule West End trips at short notice when a forecast of light easterlies and low swell lines up. Shark Cave lies roughly 1 km off the West End; charters locate it by GPS and drop divers on the reef.

There are no moorings, so private boats must anchor on sand clear of the reef or live-boat.

Highly exposed to swell, wind and current - Bucket List Diver says it is only diveable on excellent days. Needs low swell (ideally under 1 m), light easterly winds and preferably neap tides. Perth Scuba notes grey nurse sharks are most often seen in autumn.

A south-westerly sea breeze quickly makes the site and the long run home rough.

Descend directly onto the reef top at about 18 m, drop to the arc-shaped cave at 24-26 m and settle on the sand outside the entrance to watch the sharks without blocking the opening. Leave with a gas reserve for a live-boat or DSMB ascent in current.

Nitrox (EAN28-32) extends bottom time usefully; carry a DSMB for drifting ascents.

Perth Scuba and Bucket List Diver run it on exceptional-weather West End trips (a few times a year, mostly late summer and autumn); UWA Underwater Club from private boats.