The Gullies, also called the Underwater Wilderness, is a reef of cracks, gullies and boulders about 550 m south of Barrens Hut, and one of the most intricate sites in Royal National Park. The sand sits at about 22 m, with low rocks and reef fingers extending east across it. The reef rises gradually to 18 m, where a wall climbs to 14 m, then continues shallowing into large boulders up to 5–7 m.

The defining feature is a major gully running north–south, three to five metres wide and four to five metres deep, with its floor at about 10–12 m and many smaller gullies branching off. Among the boulders are overhangs, small caves and swim-throughs. The most unusual feature is The Marble: a cauldron-shaped hollow in the rocky bottom holding a huge round rock that rolls around inside when seas are big, reached through an upside-down V-shaped small cave at 14 m.

Marine life is excellent. Over ten species of nudibranchs occur on the reef tops, along with weedy sea dragons, red morwong, Port Jackson sharks and wobbegongs – up to a couple of hundred Port Jacksons in August–September. Fish schools are dense: thousands of one-spot pullers, girdled parma, yellowtail and seapike, plus black reef and six-spined leatherjackets, blue gropers, Maori wrasse, black drummer and eastern rock blackfish.

On one dive more than 50 large yellowtail kingfish circled McFadyen's group for four minutes. The route through the gullies takes navigation skill, and the anchor must be on the 18–22 m slope, not the shallows. It suits Advanced divers who enjoy intricate terrain and good navigation, on a calm day.

Boat only. Launch in Port Hacking (Gunnamatta Bay/Tonkin Park at Cronulla, Lilli Pilli or Bundeena ramps), run out through the entrance and turn right (south), passing between Jibbon Bombora and Jibbon Head in suitable weather, then follow the Royal National Park cliffs south. The site is about 550 m south of Barrens Hut, 10 minutes from the entrance.

McFadyen's GPS is 34° 05' 40" S 151° 10' 04" E on the older AUS66 datum (convert – roughly 200 m offset from WGS84). Head to the mark until the reef appears on the sounder and anchor on the slope in 18–22 m – not shallower, because in medium swell waves can almost break over the reef top.

The Royal National Park coast south of Jibbon faces east and takes swell from the north-east round to the south-east; southerly swell partly wraps around. These sites lie close to the cliffs, and the shallower ones need calm seas so that boats can anchor near shore and divers are not caught in surge – plan on seas under about 1–1.5 m. Westerly and north-westerly winds give the best conditions; a strong north-easter makes the return to Port Hacking rough.

Visibility is generally good because the coast is far from urban runoff, typically 12–20 m, occasionally 30 m. In medium swell waves can almost break on the shallow parts, and The Marble rolls in its cauldron when seas are big – a clear sign to stay away. Plan for flat seas.

Port Jackson sharks gather in large numbers in late winter (August–September).

From the anchor at 18–22 m head east across the sand between reef fingers for 12–15 minutes, then follow the wall south-west at 17–19 m to the upside-down V cave (The Marble) at 14 m. Exit left and up, go west through a notch into a narrow crack, drop to 8 m into the main gully and follow it north (about 35 minutes in) among boulders, caves and swim-throughs, then return south at 17–19 m to the anchor.

No regular charter – private boat or dive-club (St George Scuba Club) only; no charter boat currently operates out of Port Hacking.