Admiralty is Osprey Reef's home base. It lies by the southern entrance to the lagoon, where liveaboards anchor overnight, and it is often the first and last dive of each day at the atoll. Underwater Atlas calls it the primary staging area. Spirit of Freedom describes a 1,000 m-plus drop with caves, gullies and an old anchor, and Mike Ball describes towering bommies with a tunnel that holds a lodged anchor, above a sheer wall that attracts sharks and occasional manta rays.

The anchor gives the site its name. Dive the World describes it as a centuries-old admiralty-pattern anchor sitting in a cave, and marinelife.org.au notes a swim-through containing an old anchor. Its origin is not documented in the sources consulted; several ships were wrecked on Osprey in the nineteenth century, including the German steamers Freya (1882) and Papua (1885), but there is no published link between those wrecks and the anchor.

The topography is a coral 'village' of big bommies divided by sand channels, pinnacles and mounds, with the wall falling away beyond. Dive the World quotes depths of 15–60 m and visibility often above 60 m. Nearby, a cave holds flashlight fish, whose bioluminescent patches glow when divers switch off their torches, a highlight of the night dive.

Fish life is strong. Potato cod weighing more than 50 kg approach divers, according to Underwater Atlas, and midnight snapper, wrasse, hogfish, giant trevally, dogtooth tuna, barracuda, bumphead parrotfish, eagle rays and grey and whitetip reef sharks all appear. Eels, octopus and lionfish are common. Admiralty suits intermediate divers and is one of the best night dives at Osprey.

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. Admiralty is next to the usual overnight anchorage inside Osprey's southern entrance, so it is dived straight from the liveaboard, often in the late afternoon and again at night.

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. Admiralty sits by the sheltered anchorage and is one of the most dependable Osprey sites, though nutrient-rich currents can rise against the wall.

Descend among the towering bommies to about 20 m and find the swim-through with the lodged anchor. Explore the sand channels and the flashlight-fish cave (lights off for a moment at night), then look over the wall edge for sharks before returning through the bommies to the anchorage for a safety stop.

Mike Ball Dive Expeditions (Spoilsport) – 4-night Osprey fly-dive, 5-night Coral Sea & Cod Hole and 7-night Coral Sea expeditions from Cairns, several a month; Spirit of Freedom – 4- and 7-night trips including Osprey, weekly. Mike Ball lists it as Admiralty Anchor.