Blue Lagoon on Thetford Reef is a sheltered lagoon enclosed by coral, with bommies around its margins and shallow sand patches in the middle. Silverswift gives the maximum depth as 18 m with visibility around 10 m, reflecting the calmer, slightly more turbid water typical of lagoons, and it is one of the two Thetford sites Pro Dive Cairns features. It is an easy, forgiving dive.

There is no current to speak of, the depth is modest, and the bommies provide constant reference points, so it is ideal for newly certified divers, refreshers and training dives, and for snorkellers who can float over the bommie tops. Giant clams are one of the attractions; Thetford is known for very large specimens and several sit on the lagoon floor. The bommies host butterflyfish, damsels, angelfish, wrasse and parrotfish, with emperors and surgeonfish grazing the margins, and blacktip reef sharks patrol the lagoon, sometimes in the shallows where snorkellers can see them.

On the sand look for blue-spotted rays, goatfish and gobies, and in the coral for anemonefish and Christmas tree worms. Because the water is shallow and bright, the site suits photographers, although the visibility is not up to the outer walls. It works well as the final dive of the day or as a night dive in calm weather.

Note that Hastings Reef also has a site called Blue Lagoon; the two are unrelated. On a sunny day at high tide the lagoon lives up to its name, with pale sand and clear blue water, and it is one of the best places on the reef for a relaxed, unhurried hour underwater.

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.

Blue Lagoon is a regular Silverswift and Pro Dive Cairns mooring on Thetford. Enter from the stern platform into the lagoon.

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. The lagoon is one of the most sheltered moorings on Thetford and is used when stronger winds rule out the outer sites; visibility is lower than on the reef edge, typically around 10 m, and drops after rain.

Descend to the lagoon floor at 6-10 m and circuit the ring of bommies, detouring to the deepest bommie (to 18 m) if the group is comfortable. Spend time over the giant clams on the sand before a safety stop in the shallows.

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