The Opera House appears in Samphire Rottnest's guide to the island's dive sites as an offshore reef about 5 km south of Rottnest, with depths to 31 m. It is described as a sloping limestone reef with a maze of caves and swim-throughs and dramatic cavern lighting, with bullseyes, wrasse and grey nurse sharks among the fish recorded, and is rated as an intermediate dive that requires a boat.

The name suggests the kind of vaulted, chambered limestone architecture that characterises the best Rottnest reefs, such as Crystal Palace and Cathedral Rocks, and the presence of grey nurse sharks - normally associated with the West End - makes it an interesting prospect. Because it slopes to 31 m, divers can choose their depth, spending most of the dive in the cave maze at moderate depth.

Divers interested in it should ask Perth charter operators or clubs, who may know it under another name. It suits Advanced divers comfortable with depth and overhead swim-throughs.

If you do dive it, record the position and details: information on offshore Rottnest reefs is scattered, and every good account helps other divers. Carry a torch for the caverns and a DSMB for the open-water ascent. Treat depths as indicative and plan conservatively: an unfamiliar offshore reef in open water, with no mooring and no published profile, deserves a dive plan with generous gas reserves and a clear turn-around rule.

Boat only - roughly 5 km south of Rottnest and 20-25 km from Fremantle or Woodman Point (about 45-60 minutes). No published position; go with an operator or local who knows the site. Rottnest lies about 18 km west of Fremantle; allow 45-90 minutes each way by charter or trailer boat depending on the site.

Follow Rottnest Island Authority mooring and anchoring rules - do not anchor on reef or in sanctuary zones - carry a marine radio and log a trip plan, as the crossing of Gage Roads can turn rough when the afternoon sea breeze arrives.

Open water south of the island, exposed to southerly and south-westerly swell; dive on calm mornings with northerly or easterly winds. General pattern at Rottnest: summer brings calm morning easterlies followed by a strong south-westerly sea breeze from late morning, so dive early; winter brings north-westerly fronts and large swell, with clear, calm spells between them. Check the Bureau of Meteorology coastal forecast for swell height and period before committing.

Descend onto the upper slope, explore the cave maze and swim-throughs at moderate depth, working down the slope only within planned limits, then ascend on a DSMB. Agree a turn pressure before descending (half gas, or a third where current is likely), carry a DSMB, and make a 3-minute safety stop at 5 m.

Listed by Samphire Rottnest among island dive sites; no operator schedule found - ask Perth charter operators. Nearest dive services: Perth Scuba, Bucket List Diver and Dolphin Scuba (Perth/Fremantle) run Rottnest boat trips; there are no dive shops or air fills on the island.