Meet the site
Cathedral, often written Cathedrals, is Thetford Reef's best-known site and the one most often named alongside Tusa Canyons as the reef's highlight. Thetford is a long reef, stretching almost 2 km, made up largely of scattered coral bommies rising from a sandy floor at around 25 m, with sandy channels between them. At Cathedral the bommies are tall and closely spaced, and the gaps, arches and swim-throughs between them, with light streaming down from above, give the site its name. Silverswift describes coral outcrops and small bommies over a sandy seabed with an average depth of about 10 m, and an outer bommie decorated with gorgonian fans that reaches down to around 30 m. The sandy bottom reflects plenty of light, which makes the site bright and good for photography, and the coral is a mix of hard forms, including big boulder and plate colonies, with soft corals on the bommie flanks. Schools of diagonal-banded sweetlips shelter in the gaps, and turtles, barracuda and bull rays are regularly seen. Thetford is also known for giant clams several metres across, for Napoleon (Maori) wrasse, moray eels, octopus and nudibranchs, and the gorgonians on the outer bommie host small critters for macro photographers. Cairns Dive Adventures notes occasional sightings of dwarf minke whales and even whale sharks in winter. The site suits Open Water divers who stay among the inner bommies, while the outer bommie gives experienced divers a deeper option. Thetford lies about 45 km from Cairns, a little closer than Flynn and Milln, so it is often the first or last reef of a Silverswift day, and Cathedral is one of the sites guides most like to show first-time visitors.
What you might see
Diagonal-banded sweetlips, green turtles, barracuda, bull rays, Napoleon wrasse, giant clams, moray eels, octopus, nudibranchs, coral trout, gorgonian fans; dwarf minke whales (winter, occasional)
Location
Mooring on Thetford Reef, about 45 km east-south-east of Cairns, among tall coral bommies and outcrops over sand, with an outer bommie dropping to around 30 m.
Facilities nearby
Departs Reef Fleet Terminal / Marlin Marina, Cairns: toilets, cafes, paid parking; dive gear, tanks and lunch supplied on board
Before you make plans
Conditions, access and operator schedules can change. This is a location guide, not a dive plan. Ask a local operator about current conditions and suitability for your training and experience.
Sources & further reading
silverseries.com.auprodivecairns.comcairnsdiveadventures.com.audive-the-world.comreeftrip.comRecord updated 27 Sept 2026. Source claims are being reviewed.