Raine Island is a small coral cay on the outer edge of the far-northern Great Barrier Reef and the world's largest green turtle rookery. Mike Ball says between 40,000 and 100,000 green turtles visit each year to nest; Scuba Diver Life cites 40,000–130,000 and reports divers seeing up to 100 turtles on a single dive, and Divernet notes up to 20,000 turtles have been counted trying to come ashore in a single night. The island itself is closed to the public, but the surrounding reefs can be dived on a handful of expedition liveaboards each year.

The turtles migrate from Papua New Guinea, Indonesia, Vanuatu, New Caledonia and northern Australia, some travelling more than 2,500 km. During the nesting season – November and December on the expedition trips – the water around the island is full of them: turtles resting on the reef, mating at the surface, and queueing offshore for their turn on the beach. The reefs themselves are pristine, with what Mike Ball calls spectacular coral formations and Divernet describes as swathes of hard corals, black coral trees and gorgonian fans.

The mass of turtles attracts tiger sharks, which are occasionally seen patrolling the reef edge; Divernet also lists great hammerheads, manta rays, whitetip reef sharks and schooling barracuda and trevally. The dive profile is typically a drift or reef-slope dive run from tenders, and currents can be strong. Divers need Advanced certification or equivalent experience.

Raine Island is a Commonwealth-protected nature reserve and a major conservation site: rangers and scientists have reprofiled its beaches to reduce turtle deaths and nest flooding. For divers, the opportunity to see turtles in such numbers is unique. The site's remoteness – days from port and hours from medical help – adds to the sense of expedition.

Liveaboard only – there are no day boats, resorts or public moorings this far north. Mike Ball Dive Expeditions' Spoilsport runs 7-night 'Northern Exposure' expeditions between Cairns (Trinity Wharf) and Horn Island in the Torres Strait each November (northbound and southbound, up to 27 dives), plus a 7-night Far North Coral Sea Exploratory round trip from Cairns in October. Spirit of Freedom runs 7-night far-north expeditions from Cairns in October–December, and Professional Dive Services' MV Kalinda has run longer (13-night) Cairns–Horn Island trips.

Horn Island has daily scheduled flights to Cairns (book separately; allow 24 hours after diving before flying). The far-northern reefs are roughly 500–650 km north of Cairns – one to two days' steaming each way.

The far-northern reefs are only diveable in a narrow weather window. The south-east trades blow hard from about April to September and make the long open-water passage and the exposed outer walls impractical, so expeditions run in the transition period, typically October–December, when winds are lighter; the northerly monsoon, storms and cyclones (December–April) close the season. Even in the window, exposed shelf-edge walls (Tijou, Great Detached, Raine) need calm days, and skippers switch between reefs to find shelter.

Strong tidal currents and upwellings run along the continental shelf and through reef openings – channel sites are timed to slack water. Visibility varies from about 12 m to 40 m+ depending on tides and any rain. Marine stingers are present from November.

Dives are run from tenders along the reef surrounding the cay, often drifting along the reef slope at 10–18 m. Stay close to the guide, expect turtles everywhere and keep watch into the blue for tiger sharks; make a group ascent with an SMB for tender pick-up.

Liveaboard only: Mike Ball Dive Expeditions (Spoilsport – 7-night Northern Exposure Cairns↔Horn Island, November; 7-night Far North Coral Sea Exploratory, October); Spirit of Freedom (7-night far-north expeditions, October–December); Professional Dive Services MV Kalinda (occasional long expeditions). Requirements typically Advanced certification or 15+ logged ocean dives.