Stonehenge on Agincourt Reef No. 3 is one of the most highly regarded coral sites out of Port Douglas. Silverseries describes it as rich in coral diversity with awesome swim-throughs, and Scuba IQ goes further, calling it one of the most incredible sites on the Great Barrier Reef, absolutely loaded with healthy coral. The name comes from the standing coral pillars and arches that give the reef its structure.

Big coral heads have grown up from the sand to around 5 m below the surface, and in places they have joined overhead to form a sequence of arches and short tunnels. Swimming through them is the highlight of the dive – they are open, lit from the sides and short enough to suit any diver with good buoyancy control. Inside, the walls are lined with sponges, soft corals and small fish sheltering from the light.

Around and between the pillars the coral cover is dense and varied: staghorn and plate Acropora, massive boulder corals, brain corals, soft corals and sea whips. The fish life matches it. Fusiliers swirl overhead, clownfish nestle in anemones on the tops of pillars, and cuttlefish hover near coral heads.

Giant clams are dotted across the reef, and reef sharks are often seen cruising the sand edges. The maximum depth is around 20 m, but most of the dive takes place between 8 and 15 m, which keeps it suitable for new open water divers and leaves plenty of light for photography. Scuba IQ notes it appeals to a wide range of diver abilities.

Stonehenge is a site to take slowly. Rather than covering distance, spend time weaving through the swim-throughs and examining the coral surfaces, where nudibranchs, flatworms and shrimps reward close inspection. It is one of the sites guides most often recommend when asked for the best coral on Agincourt.

Agincourt day boats leave Crystalbrook Superyacht Marina (formerly the Reef Marina) on Wharf Street, Port Douglas, between about 8:00 and 8:30 am. Park in the marina car park or the free all-day street parking around Wharf and Dixie streets; Quicksilver, Poseidon and Calypso also run coach pick-ups from Port Douglas, Palm Cove and northern Cairns beaches. The run to the Agincourt ribbon reefs on the edge of the continental shelf is about 65–70 km north-east and takes roughly 75–90 minutes on the large catamarans (Silversonic is the fastest), longer on small-group boats such as Scuba IQ/Blue Dive.

You gear up on board and giant-stride from the dive deck at a permanent mooring; the skipper picks the sites on the day to suit wind and tide, so a named site is never guaranteed. Boats are back at the marina around 4:30 pm.

Agincourt is an outer ribbon reef on the shelf edge, so it is fully exposed to the south-east trade winds that blow from about April to September. Moorings on the protected western (back-reef) side remain diveable in 15–20 knots, but anything over 25 knots or a 2.5 m+ sea makes the crossing rough and operators may cancel or switch to more sheltered moorings. Calm spells in October–December give the glassiest days and 30 m+ visibility.

The wet season (December–April) brings northerlies, afternoon storms, cyclone risk and occasional reduced visibility after big rain events, although the outer reef is far less affected by run-off than inshore reefs. Marine stingers are present November–May. Tidal currents run along the reef edges and through the gaps between the Agincourt reefs; spring tides strengthen them, so exposed corners are usually dived on the neaps or near slack water.

Follow the guide from the mooring through the coral structures at 10–15 m, taking the swim-throughs in single file, then loop back over the reef top at 6–8 m searching for clams, cuttlefish and anemonefish. Safety stop on the reef top by the mooring.

Quicksilver Silversonic – daily, 3 dives across Agincourt 1–4; Poseidon (Quicksilver Group) – daily, 3 sites at Agincourt; Calypso Reef Cruises – regular Agincourt days; Scuba IQ / Blue Dive and Tech Dive Academy – small-group day trips several days a week.