The Swains are the most remote reefs of the southern Great Barrier Reef: around 1,000 reefs spread across the outer shelf, separated by channels 30-80 m deep, with the continental slope dropping to about 1,000 m just offshore. Diving is by liveaboard or private yacht only, and divers must be self-sufficient, with no assistance nearby. Mystery Cay is described by the Marine Life Network as having extensive coral gardens with pinnacles at around 20 m, where manta rays visit cleaning stations during the winter months.

That makes it one of the Swains' best manta sites, although winter is also when trade winds most often limit trips to the area. Around the pinnacles, divers can expect reef fish, coral trout, cod, whitetip reef sharks and sea snakes, and the coral gardens between them offer good general reef diving. The site suits Open Water divers with some experience.

It was grouped with East Cay and Lavers Cay in the earlier database pass; all three are now listed separately. The Swains lie within the Great Barrier Reef Marine Park under a mix of zones, and the nine vegetated cays of Swain Reefs National Park are among Queensland's most important seabird breeding sites and significant for endangered loggerhead turtles; most are closed to landing year-round. Anchor only on sand, check the zoning map before fishing, and carry oxygen, a satellite communicator and a detailed evacuation plan, since aeromedical retrieval to Mackay and a chamber in Townsville takes many hours.

The Swain Reefs are the most remote part of the southern Great Barrier Reef, roughly 200-250 km offshore. Liveaboards depart Gladstone Marina or Rosslyn Bay Harbour (Yeppoon) in the evening and steam overnight across the 50-mile-wide Capricorn Channel, a passage of 12-15 hours in good weather; trips run 6-10 nights. Private yachts should carry detailed charts (reefs are separated by 30-80 m deep channels) and plan for no outside assistance.

Once in the Swains, liveaboards move between reefs by day and anchor in the lee of a reef or cay at night; this site is visited as part of a multi-day itinerary.

The Swains are fully exposed. South-east trade winds of 20-30 knots, common June-August, make the Capricorn Channel crossing rough and restrict diving to the lee (north-western) sides of reefs; charter operators generally shelter behind reefs at anchor in 20-25 knots. Best windows are the calm spells of September-December and March-May.

Cyclones can reach the area November-April. Tidal currents run hard through the channels between reefs, so outer-edge sites are dived as drifts near slack water; lagoon and lee sites work in most tides.

Descend among the pinnacles, find a cleaning station and settle nearby to wait for mantas in winter, then explore the surrounding coral gardens before ascending.

No day operator. Kanimbla Charters runs 7- and 10-day dive and fishing charters to the Swains aboard a 25 m catamaran (all tanks and weights supplied); Eastern Voyager runs 6-7 night trips from Gladstone mainly for fishing, snorkelling and spearfishing. Most diving here is from private yachts and occasional expedition charters.