Phil's Backside is a Scuba IQ favourite on the Agincourt complex, described by the operator as a site of deep bommies and a massive coral slope that is home to the biggest giant clams they have ever seen. Given how many clams Port Douglas guides see in a year, that is a strong recommendation. The dive is built around a broad slope of hard coral running down from the reef edge at about 5 m into deeper water, with isolated bommies standing off the bottom of the slope.

The slope is densely covered – plate, staghorn and boulder corals interlock across it – and it holds a big population of reef fish: fusiliers and chromis above, parrotfish and surgeonfish grazing, and wrasse picking through the coral. The deeper bommies attract larger fish and are worth visiting early in the dive while you have plenty of gas and no-decompression time. The clams are the headline.

Giant clams (Tridacna gigas) can exceed a metre across and live for decades; the ones here are huge, their mantles patterned in greens, blues and browns. Watch them close slowly as your shadow passes over, and resist the urge to touch – they are protected and sensitive to disturbance. Smaller Tridacna species with brighter mantles are scattered across the slope too.

The name is a play on neighbouring Phil's Reef on Agincourt 3, and the site is on the same part of the complex; operators do not publish a precise depth range, so expect a slope running from the shallows to well beyond 20 m, with the dive normally planned in the 10–20 m band. It suits divers comfortable at moderate depth and anyone who enjoys long, relaxed swims over healthy hard coral. As one of the less frequently visited moorings, it tends to feel quiet even on busy days.

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.

Descend onto the coral slope and head down to the deeper bommies first (hold no deeper than 18–20 m unless advanced), then work back up the slope diagonally, scanning for the giant clams. Finish at 5–6 m on the upper slope for the safety stop.

Scuba IQ / Blue Dive – small-group day trips to Agincourt several days a week; occasionally used by other Port Douglas day boats.