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.

Boat access only. Cairns day boats leave from the Reef Fleet Terminal (1 Spence Street) and the neighbouring Marlin Marina on the city waterfront, with check-in between about 7:00 and 8:00 am; most operators run transfers from city and northern-beaches accommodation, and there is paid parking near the terminal. The run of about 45 km out through Trinity Inlet and across the lagoon to Thetford Reef takes around 80 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Cathedral is one of the two Thetford sites Pro Dive Cairns features and a regular Silverswift mooring. Enter from the stern platform and descend the mooring line.

The prevailing south-east trade winds (typically 15-25 knots from April to September) are what limit Cairns outer-reef diving: above about 25 knots trips may be cancelled, and exposed eastern moorings are used only in light winds. Summer northerlies are gentler but make north-facing moorings choppy. Box jellyfish and Irukandji are present from about November to May (stinger suits on every dive), and the cyclone season runs November to April, when a system in the Coral Sea can close the reef for days.

Visibility drops after heavy rain and strong winds; spring is usually calmest and clearest. Thetford is a little closer to the coast than Flynn and Milln and can lose visibility after wet-season rain; Cathedral is used in normal trade-wind conditions and can carry current between the bommies on spring tides.

Descend the mooring to the sand at about 12 m, head first to the outer bommie with its gorgonian fans (dropping to 20-25 m if your plan allows), then work back through the cathedral-like gaps and swim-throughs between the inner outcrops at 8-12 m. Safety stop on a bommie top at 5 m.

Silverswift – regular day trips (Thetford is one of its four core reefs); Passions of Paradise – day trips to 360 and East Meets West; Pro Dive Cairns – 3-day liveaboard includes Thetford; Tusa Reef Tours – occasional