Crystal Palace is the best known reef dive at Rottnest and the one most Perth divers name first. It takes its name from the way bubbles escaping through holes in the limestone roof glitter like crystals, and from the cathedral-like chambers below. Marinelife.org.au describes a stretch of caves and swim-throughs 600-800 m long south-west of Dyer Island; Samphire places it east of Parker Point about 850 m from the island, and the Rottnest Island Authority describes it as on the south-east side. (Some guides say south-west, but the published coordinates put it off Porpoise Bay.)

The reef top sits at about 8 m and drops into chambers and swim-throughs to around 18 m, with light shafts cutting down through openings in the ceiling. The overhead sections are open caverns rather than true caves, but the network is extensive enough that divers can spend a whole dive moving from one chamber to the next. Perth Scuba describes 'endless caves and swim-throughs' with big schools of buffalo bream of 50-60 cm, snapper, dhufish, western rock lobster and wobbegong and Port Jackson sharks, and turtles on night dives. Marinelife.org.au lists harlequin fish, western blue groper, samson fish, western blue devils, morwong and boxfish, and Bucket List Diver notes a range of nudibranchs on the walls.

It suits Open Water divers comfortable with buoyancy control in overhead swim-throughs; it is a superb photography site and one of the reasons Rottnest diving is so highly regarded. Bring a torch for the darker chambers, and allow two dives to see more than one section of the reef.

Charter boats from Fremantle (Perth Scuba's Blue Destiny, Bucket List Diver, Dolphin Scuba) run across Gage Roads and round the east end of Rottnest to the south coast in roughly 50-70 minutes. Private boats launch at Fremantle Fishing Boat Harbour, Woodman Point or Hillarys; Crystal Palace is outside Porpoise Bay south-west of Dyer Island, about 20-22 km from Fremantle; approach carefully around the South Passage reefs and anchor on sand clear of the reef or use the charter's drop. Charters leave Fremantle around 8 am and return mid-afternoon with two dives.

On the south-east side of the island, so it is sheltered from westerly swell but exposed to southerly swell and wind; charters use it in northerly and easterly conditions. Surge in the chambers increases with any southerly swell. Winter (cooler season) brings clearer water and better light penetration according to Dolphin Scuba; December-April gives warm, settled weather.

Descend onto the reef top at 8-10 m, pick a section of the reef and work along it through the swim-throughs and chambers at 12-18 m, keeping the exits in sight; turn at half gas and return along the reef top for the safety stop.

Signature site for Perth Scuba (Blue Destiny day trips from Fremantle), Bucket List Diver and Dolphin Scuba (winter); UWA Underwater Club; dived most weeks in suitable weather.