The 20 Metre Ridge is an offshore reef north of Rottnest described by marinelife.org.au as a ridge at 20 m surrounded by a flat reef at 32 m, a couple of kilometres offshore. It is one of the deeper reef sites listed around the island, and the kind of site used by experienced Perth divers looking for bigger fish away from the busy inshore reefs.

Offshore ridges like this tend to attract demersal species such as dhufish and pink snapper, along with samson fish and kingfish in the water column, and the reef surfaces carry sponges, gorgonians and encrusting growth that are better developed than on shallow, surge-swept reefs. Visibility is often good because of the distance from shore.

With a minimum depth of 20 m and surrounding reef at 32 m, the site requires deep training or experience and careful attention to no-decompression limits; nitrox is recommended for the ridge and air or suitable nitrox for any excursion deeper.

It suits advanced divers with their own boat who are comfortable in open water and want a quieter, deeper reef dive within an hour of the metropolitan boat ramps. Leave a competent person on the boat, carry a DSMB and plan for current, which can pick up quickly on offshore reefs. Two short dives are better than one long one here, with a long surface interval between them.

Private boat from Hillarys, Fremantle or Woodman Point (about 20 km, 40-50 minutes). Locate the ridge by GPS and sounder and anchor on the ridge edge or live-boat. 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 north of the island, exposed to westerly and north-westerly swell and to wind chop. Needs a calm day; summer mornings are best. 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 the anchor line to the ridge at 20 m, follow the ridge along its length, dropping to the flat reef at up to 32 m only within planned limits, then return to the line. 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.

Nitrox recommended; watch no-deco limits.

No regular operator - private boats and dive clubs (e.g. UWA Underwater Club) only. 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.