North Point Reef is a deeper Rottnest dive built around a single striking feature. The UWA Underwater Club describes a unique limestone structure forming a trench about 8 m deep and 3-4 m wide, cut into the top of the reef, with sunlight rays visible through the valley above; average depth is around 18 m and maximum 26 m. Marinelife.org.au describes the same site as a large ravine within the top of a mound, the ravine bottom lying at 26 m, and notes that the ground to the north at 20-25 m is also worthwhile.

Swimming through the trench is the highlight: walls rise on either side, light filters down from the narrow opening, and fish shelter in the cracks. The deeper setting and distance from shore mean bigger fish than on the inshore reefs - buffalo bream, western blue groper, dhufish and samson fish are all possible - and the walls carry sponges and encrusting life. Nearby North Point Hole is a separate cavern site at a similar depth.

The site is demanding because it requires calm seas and precise navigation to find the ravine, and because depths of 20-26 m limit bottom time on air. It suits Advanced divers with nitrox who want something more dramatic than the shallow north-east reefs. Plan it for a flat day and combine it with a shallow second dive such as the City of York wreck. Because the ravine is narrow, keep the group small and move through it in single file, and agree on an exit direction before entering so that nobody turns back into oncoming divers. Photographers should shoot upward from the floor to catch the light rays.

Charters and private boats from Fremantle, Woodman Point or Hillarys run along the north coast to North Point (about 25 km, 60-70 minutes). Marinelife.org.au recommends an echo sounder to locate the ravine in the top of the mound; anchor or live-boat nearby. 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.

Needs ideal conditions: light winds and swell under about 1.5 m (marinelife.org.au). Exposed to north-westerly and westerly swell; best on calm summer and autumn mornings. Sunlight rays through the ravine are best around the middle of the day in clear water.

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 mound at about 18-20 m, find the ravine and drop into it, swimming its length at up to 26 m under the light rays; exit onto the top and explore the northern ground at 20-25 m before ascending.

Nitrox recommended.

A UWA Underwater Club favourite; occasional Perth charter site when the north coast is calm. 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.