The Outer Transit Caves are the natural-reef counterpart to the famous wrecks on Transit Reef. The seaward face of the reef is undercut into limestone swim-throughs, arches and overhangs in shallow water, and the marinelife.org.au site guide notes that visibility here is often excellent, sometimes better than on the wrecks just inside the reef. The structure is typical north-coast Rottnest limestone: ledges and caves with sponges and colourful encrusting invertebrates on the walls and kelp on the reef top.

Fish life is good because the area lies within or beside the Kingston Sanctuary Zone. Expect leatherjackets, wrasse, bullseyes packed into the darker holes, and schools of bream over the reef, plus western rock lobster under ledges. The shallow depth gives long bottom times, and the caves are open enough that Open Water divers can enjoy them without true overhead penetration.

This is a good second dive for charter groups who have already done the wrecks, for divers wanting natural reef close to Thomson Bay when the south side is blown out, and for photographers who want light shafts through the swim-throughs. It is close to the Thomson Bay jetty and the main boating channel, so good surface discipline is essential. Because published information on the site is thin, go with someone who knows the reef or allow time to explore: the best caves are on the outer face rather than the reef top. A torch brings out the colour in the sponges and helps find lobster and cuttlefish tucked into the back of the overhangs. Stay clear of the yellow glass-bottom-boat mooring on the nearby wrecks when manoeuvring.

Rottnest lies about 18 km west of Fremantle. Perth charter boats (Perth Scuba's Blue Destiny and Bucket List Diver) leave Fremantle in the morning and reach Thomson Bay in about 45-60 minutes; the Transit Reef wrecks are usually picked up on a mooring just outside the bay as a second, shallow dive. Private boats launch at Fremantle, Woodman Point or Hillarys and should use a public mooring rather than anchor inside the Kingston Sanctuary Zone.

It is also possible to catch the Rottnest Express or SeaLink ferry to the main jetty and kayak or make a long surface swim north from the Thomson Bay beach, but the route crosses a busy boat channel and a tow float and flag are essential.

Protected from the prevailing south-west swell and sea breeze; exposed to north-westerly winds and winter storms. Often clear water. Best under 1.5-2 m swell with easterly winds; heavy boat traffic nearby.

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 on the reef edge, work along the outer face through the swim-throughs and under overhangs at 5-8 m, then turn back along the reef top. Can be linked to the Macedon mooring on a compass bearing. 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.

Occasional stop for Perth charter boats (Perth Scuba, Bucket List Diver) and dive clubs as an alternative or add-on to the Transit Reef wrecks.