Swirl Reef is one of the most unusual formations around Rottnest. Marinelife.org.au describes it as formed like a crater with a hollow in the centre, riddled with swim-throughs and with big caves on its north-west and south-east sections - a particularly interesting piece of geology even by the standards of the island's eroded limestone. Bucket List Diver's account centres on a massive cave with overhead gaps that let in shafts of light, entered at about 15-17 m, with the crater behind it and deeper sections beyond.

The name refers to the swirl that forms on the surface after each swell passes over the reef, which also tells you what happens underwater: surge moves through the cave and swim-throughs, so the site is best on flat days. When conditions allow, it is a spectacular dive, with the combination of light beams, an enclosed crater and multiple exits giving a sense of exploring a structure rather than a reef. Fish life includes bullseyes and old wives in the shaded areas, buffalo bream and wrasse over the rim, and western rock lobster in the cracks.

The dive suits Advanced divers comfortable with overhead swim-throughs and surge, although Open Water divers can manage the main cave on a calm day. It is a good choice when the north-west of Rottnest is calm but the West End is still too rough, and it pairs with City of York or North Point on the same trip. A torch helps in the darker recesses. Check the surface before entering: if the swirl is violent, choose another site.

Charter boats from Fremantle (Bucket List Diver, Perth Scuba) run along the north coast to the north-west of the island, about 25 km and 60-75 minutes. Private boats launch at Fremantle, Woodman Point or Hillarys. Locate with GPS and sounder (the reef shows as a ring on the sounder) and anchor in sand clear of the reef.

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.

Often exposed to surge and highly weather-dependent (Bucket List Diver). Needs low swell from the west and north-west and light winds, usually summer and autumn mornings. The name comes from the swirl seen on the surface after a swell passes over the shallow reef, a reminder that surge inside the cave can be strong.

Descend to the main cave entry at 15-17 m, pass through the large cave under the light gaps, then explore the crater behind it and the caves on the north-west and south-east sides; return along the rim and ascend from the reef top.

Bucket List Diver runs it when the north-west coast is calm (Advanced preferred; Open Water divers can do the main cave); Perth charters and clubs from private boats occasionally.