Acropolis is a site on Opal Reef that Scuba IQ describes as a shallow coral theatre leading to a wall that circles toward the northern channel sites, and recommends for nudibranch hunting. The coral theatre is a shallow, amphitheatre-like area of hard corals – the name nods to Acropora, the branching and plate coral genus, as well as to the ancient Greek citadel. It is bright, colourful and full of reef fish.

From there, a wall curves around the reef towards the channels at the northern end, offering a different habitat with overhangs, crevices and encrusting life. The wall is where nudibranchs are found. These colourful sea slugs graze on sponges, hydroids and ascidians, and a slow search along the wall can turn up several species.

Other macro life – flatworms, shrimps and small crabs – is also common. Scuba IQ does not publish depths; the shallow theatre is probably within 3–8 m, while the wall likely extends to 15 m or more. Visibility is typically 15–30 m.

The site suits all levels, especially those interested in macro life, and is best dived slowly. Acropolis offers a combination of shallow coral scenery and macro-rich wall diving, and its connection to the northern channel sites means guides can adapt the route to conditions. Divers heading towards the channel should stay close to the guide, as current can pick up near the gaps between the Opal reef sections.

Boats to Opal Reef leave Crystalbrook Superyacht Marina on Wharf Street, Port Douglas; Calypso Reef Cruises checks in around 8:15 am for a 9:00 am departure and returns about 4:30 pm, and Wavelength and Scuba IQ/Blue Dive leave around 8:00 am. Park in the marina car park or on surrounding streets (free all-day parking along Wharf/Dixie streets), or use the operators' courtesy pick-ups from Port Douglas and the northern beaches. Opal is about 60 km north-east of Port Douglas; the crossing takes 80–90 minutes, part of it threading through the protected Tongue Reef maze.

Divers gear up on board and enter by giant stride at a mooring; on Calypso some sites are reached by tender from the main vessel. The skipper chooses three sites on the day depending on wind and tide.

Opal is a crescent-shaped outer reef near the continental shelf edge. Its shape gives some shelter from the south-east trades that blow from about April to September, so several moorings (Bashful, Sno, Blue Buoy) stay diveable in 15–20 knots, but a 25-knot-plus trade wind makes the crossing rough and operators may cancel. Exposed sites on the seaward face and in the channels between North, Middle and South Opal are dived only in light winds and suitable tides.

October–December is typically calmest with the best visibility; the wet season (December–April) brings northerlies, storms, cyclone risk and marine stingers (November–May). Tidal currents run through the gaps between the Opal reef sections and around bommies, strongest on spring tides – channel and bommie sites are timed for neaps or slack water.

Start in the shallow coral amphitheatre at 5–8 m, then follow the wall as it curves towards the northern channel at 10–15 m, searching the wall for nudibranchs. Turn at half gas and return shallow for the safety stop.

Calypso Reef Cruises – daily Opal trips (3 dives/snorkel sites); Wavelength Reef Cruises – snorkel-only daily trips to exclusive Opal moorings; Scuba IQ / Blue Dive and Tech Dive Academy – small-group dive days several times a week; private charters.