Three Mile Opening – TMO to the crews – is a far-northern site visited on Mike Ball's Far North Coral Sea Exploratory and Raine Island expeditions. Mike Ball describes upwelling currents that attract schooling trevally, barracuda, mackerel, tuna and sharks, with mantas and eagle rays at the continental shelf; Scuba Diver Life notes TMO features bommie maze formations. Openings are gaps in the outer barrier through which tidal water moves between the Coral Sea and the lagoon behind the reef.

At the shelf edge these flows create upwellings, drawing nutrient-rich deep water to the surface and triggering plankton blooms that attract fish. The result is some of the most pelagic-rich diving on the reef: big schools of trevally and barracuda, hunting mackerel and tuna, and sharks cruising the edges. The bommie maze gives divers structure to shelter behind when the current runs, and the gaps between bommies create interesting navigation.

Mantas and eagle rays are drawn to the plankton. Current is the main consideration, and operators time dives carefully. The site requires Advanced certification or equivalent experience.

TMO suits experienced divers looking for pelagic action in a remote setting. It is typically dived on the same day as Creech Reef on Mike Ball's exploratory itinerary. Getting here is itself an expedition: the far-northern reefs lie hundreds of kilometres north of Cairns, are visited by only a handful of liveaboards in the October–December window, and are many hours from medical help, so divers should be current, self-sufficient, carry an SMB and audible signal, and dive conservatively.

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.

Timed for manageable current: drop in with the guide near the opening, work through the bommie maze at 15–20 m watching the blue for schooling pelagics and mantas, and ascend together 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.