Cathedral Rocks is one of those Rottnest sites that divers wait years to do. At the far West End, near Cape Vlamingh and the fur seal colony, a group of rock stacks and outcrops has been hollowed into a network of caverns. Perth Scuba describes a massive cave whose roof has 'windows' that throw beams of light into the chamber like a cathedral, linked to a series of smaller caverns; Divernet likens the effect to disco laser lights. Marinelife.org.au describes a collection of large caves and crevices, with rocky reef extending seaward and a main cavern about 10 by 5 m under a large rocky outcrop, noting that life inside the main cavern is limited, that a torch and ample gas are needed, and that the reefs at the bay entrance have claimed tuna boats - the Kiryo Maru I debris lies nearby.

The catch is access. The site faces straight into the Southern Ocean swell, and Perth Scuba says it needs five or six consecutive days of easterly winds in summer and is diveable only two or three times a year. When it is on, the combination of light beams, cavern architecture and seals in the water makes it one of the most memorable dives in Western Australia.

It is for Advanced divers who are comfortable in overhead environments, can handle surge and carry torches and a backup light. Depths are moderate, but the setting is remote. The surrounding waters are within the West End Demersal Sanctuary Zone. Divers should keep their distance from seals and sea lions hauled out on the rocks.

The West End is the far western tip of Rottnest, around 30 km from Fremantle; Bucket List Diver quotes 1-2 hours by charter boat from the mainland, and private boats need a long run along either coast. Only a handful of days a year are calm enough, so operators schedule West End trips at short notice when a forecast of light easterlies and low swell lines up. Boats navigate the sheltered eastern reef passage into Rottnest's Eagle Bay, where there is anchoring, and divers swim to the caverns under Cathedral Rocks.

The rocks and seals can be seen from the Cape Vlamingh boardwalk, but there is no safe shore access for divers.

Perth Scuba and Divernet say Cathedral Rocks needs five or six consecutive days of easterly winds in summer and is diveable only two or three times a year. Any significant south-west swell produces heavy surge in the caverns and breaking water over the rocks. Dive at slack tide in the morning.

Anchor in the sheltered eastern side of the bay, swim to the rocks and enter the main cavern (about 10 x 5 m under the large outcrop) with torches, look up at the light beams through the roof windows, then explore the smaller caverns and crevices seaward before returning to the bay.

Perth Scuba describes it as accessible only 2-3 times a year and runs it on those rare calm days; otherwise private boats and clubs.