Porpoise Bay Caves are a small companion site to Crystal Palace on Rottnest's south-east side. Marinelife.org.au lists them as a 15 m reef of small overhangs and caves, just outside Porpoise Bay and close to Crystal Palace, 'packed with marine life', with average visibility of about 10 m. The two sites share the same limestone reef system south-west of Dyer Island, so this is essentially a quieter extension of the more famous reef.

The overhangs shelter the fish you would expect on this part of the island: bullseyes, old wives and sweep in the shade; buffalo bream, harlequin fish and western blue groper on the open reef; and western rock lobster and cuttlefish in the cracks. Because fewer boats drop here than on Crystal Palace, it can be a more relaxed dive with fewer divers, while still offering the eroded limestone architecture that makes the south-east coast of Rottnest so good.

It suits Open Water divers comfortable at 15 m. There are no true caves requiring training - the overhangs are shallow and open - but a torch helps to see into them. The dive can be run as a standalone site or as a second tank after the Lady Elizabeth wreck, which lies nearby behind Dyer Island.

Published information is limited to a short listing, so depths and features here should be treated as indicative and checked against local knowledge. Charter skippers working Crystal Palace are the best source of up-to-date advice. Southern swell days turn the site surgy, so check conditions from the boat first.

Launch at Fremantle Fishing Boat Harbour or Woodman Point and run around the east end of Rottnest to the south coast (about 20-25 km, 45-70 minutes in a trailer boat). Locate the site by GPS and echo sounder and anchor in sand clear of the reef, or live-boat. Porpoise Bay Caves lie immediately beside Crystal Palace south-west of Dyer Island, so charters sometimes use them as an alternative drop.

Approach carefully around the South Passage reefs.

As for Crystal Palace: sheltered from westerly swell but open to southerly swell and winds; best in northerly and easterly conditions on summer mornings. Average visibility is about 10 m (marinelife.org.au). 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 on the reef at about 12 m and work along the small overhangs and caves at 14-15 m, then return over the reef top; combine with Crystal Palace by swimming along the reef. 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.

No dedicated operator; occasionally dived by charters working Crystal Palace and by private boats. 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.