Nautilus Wall takes its name from Osprey Reef's population of chambered nautilus (Nautilus pompilius), one of the reef's scientific curiosities. Wikipedia notes that Osprey supports a dwarf population of the species, isolated from others by more than 100 km of deep water. Nautiluses live on the steep outer walls of reefs at depths of roughly 150–700 m, rising toward shallower water at night to feed, which means divers almost never see them in the wild.

That practice involved setting traps on the deep wall overnight, under permit, and releasing the catch at first light in shallow water, where divers can photograph the animals as they pump their way back down.

Without a nautilus, the site is a classic Osprey wall. The face drops into the abyss beneath soft corals, sea fans and whips, with grey reef and whitetip sharks, dogtooth tuna, trevally and schooling anthias. Visibility is usually excellent.

The site suits intermediate divers. Anyone keen on a nautilus encounter should ask the operator before booking, and should follow the guides' instructions closely: the animals are fragile, sensitive to warm surface water and light, and must not be handled or chased.

Liveaboard only. Mike Ball's Spoilsport runs 4-night fly-dive trips (depart Cairns Thursday evening, cruise north overnight via the Ribbon Reefs, cross to Osprey, fly back from Lizard Island) and 5–7 night Coral Sea expeditions; Spirit of Freedom visits Osprey on its 4- and 7-night itineraries, with a Cairns–Lizard Island flight connection. The open-ocean crossing from the Ribbon Reefs to Osprey (about 100 km) is made overnight.

Allow 2–3 days at Osprey on a typical trip. Nautilus Wall is listed on Mike Ball's Far Northern Reefs expedition and its itinerary map. Historically, nautilus encounters were arranged by catching animals in deep traps overnight under permit and releasing them in the shallows at dawn for divers to photograph; check with the operator whether this is still offered.

Osprey is an isolated oceanic atoll about 350 km north-east of Cairns and roughly 100 km beyond the edge of the Great Barrier Reef, so every trip involves an overnight open-ocean crossing. Strong south-east trades (over about 25 knots, common June to August) make the crossing rough and can close the exposed eastern and southern walls; liveaboards then work the sheltered western wall and North Horn. Sep–Dec usually brings the lightest winds and visibility of 40 m or more.

Currents on the walls vary with tide and oceanic flow and can run strongly around the horns. Dec–Apr is cyclone season and trips can be cancelled or diverted to the Ribbon Reefs.

If a nautilus release is run, descend at dawn to the shallow reef edge (5–15 m) with the guide, photograph the nautilus as it begins its slow descent, and do not touch or pursue it past your planned depth. Otherwise dive as a standard wall: follow the wall at 15–20 m and ascend along it.

Mike Ball Dive Expeditions (Spoilsport) – Far Northern Reefs and some Coral Sea itineraries.