Under the Tube takes its name from the waves that tube over the rocks at the north-east corner of Moon Island, and the dive is on the steep wall beneath them. Grey Nurse Charters describes a rocky reef wall from 12 to 22 m with the Slot Cave, transitioning to sand in the deeper sections, and notes a strong population of eastern blue devilfish. iNaturalist's Moon Island notes add a steep wall, the 'amazing' Slot Cave and the Donut Cave. The wall is the main attraction: sheer in places, with cracks, holes and overhangs that shelter a wide range of fish and invertebrates.

The Slot Cave is a narrow cleft cutting into the rock - a spectacular feature in good visibility - while the Donut Cave is a rounded hole that frames divers nicely for photos. Soft corals, sponges and ascidians cover the shaded surfaces. Eastern blue devilfish are the resident stars.

These shy, electric-blue fish with white spots hide in holes and crevices and are a favourite of photographers; Under the Tube has more of them than most sites in the region. Around them are bullseyes, one-spot pullers, red morwong, wrasse and wobbegongs, with weedy seadragons, bull rays and Port Jackson sharks recorded around the island. In winter, grey nurse sharks from The Temple sometimes move along the walls.

At 12-22 m the dive suits Advanced divers, or Open Water divers with a guide, who are comfortable near a wall with some surge overhead. It pairs well with The Temple or The Arch on a double-dive morning from Swansea.

Boats launch from the public ramps inside Swansea Channel, either at Black Neds Bay beside the Swansea Bridge or at the Swansea/Lake Macquarie ramps, about 25 km south of Newcastle via the Pacific Highway. Parking with trailer bays is available at Black Neds Bay. The run is east along the channel, across the Swansea bar (check the bar and tide first - cross on the flood where possible) and a short distance out to the site; Moon Island sits just off Swansea Heads (Reids Mistake), so the run from the bar is only a few minutes - total 10-15 minutes from the ramp.

Under the Tube is on the north-east corner of the island, named for the waves that tube over the rocks above it.

Moon Island and the reefs off Swansea Heads sit just outside the Swansea Channel bar, which is the deciding factor on most days: the bar is dangerous in any sizeable easterly or south-easterly swell and on a run-out tide, and Marine Rescue Lake Macquarie should be checked before crossing. Offshore, the sites prefer swell under about 1.5 m and light westerly or northerly winds; a strong nor'easter makes the eastern walls choppy, and a southerly swell pushes surge into the caves. Visibility runs 8-15 m and is best on a flooding tide, when clean ocean water replaces the lake outflow; after heavy rain the brown plume from Lake Macquarie can cover the inshore sites for days.

The north-east corner catches north-easterly wind chop and swell; it is best on a southerly or westerly day with a small sea.

Descend near the wall and follow it at 15-20 m, looking into the Slot Cave and Donut Cave and searching holes for blue devils. Where the wall gives way to sand at 22 m, turn and return along the upper wall, staying clear of the surge zone under the breaking waves.

Grey Nurse Charters (Swansea) runs boat dives to the Moon Island and Swansea offshore sites most weekends and on demand; Dive Newcastle and local clubs also dive the area from private boats launched at Swansea.