Greg's Grotto is a maze of ledges and caves covered in colourful corals, sponges and gorgonians at about 14 m, with abundant reef sharks and mackerel, per the Marine Life Network.

About the Mackerel Islands: the group of ten low islands off Onslow, including Thevenard and Direction islands, sits just north of the Ningaloo Coast and Exmouth Gulf. The Marine Life Network documents a dozen dive sites within easy reach of Thevenard, from shallow coral heads to ledges, caves and canyons at up to 20 m, and the resort notes whales, dolphins, groper, reef sharks, stingrays, wobbegongs, turtles and thick schools of trevally, snapper, fusiliers and batfish. With no resident operator and few visiting divers, the reefs are among the least dived in the region — ideal for self-sufficient divers with their own boat.

Planning notes: Open Water certification is recommended and the site is rated intermediate. Expect to spend most of the dive around 14 m, with a maximum of about 14 m. Access is by boat only, and site selection depends on wind, swell and tide on the day. Best season: April–October. Traffic: rarely dived (private boat/club only).

Onslow is about 1,380 km north of Perth, 80 km off the North West Coastal Highway. The Mackerel Islands resort on Thevenard Island is reached by the resort's ferry or charter flight from Onslow (Beadon Creek), about 22 km offshore (roughly 45 minutes by boat). There is no dedicated dive operator on the islands: divers bring their own gear and boat or hire a boat, and scuba equipment can be pre-booked through Pilbara Dive and Tours in Karratha.

Most scuba sites (3–18 m) are close to Thevenard; the house reefs and pier are accessible from the beach.

The islands sit on the shallow Pilbara shelf between Exmouth Gulf and Barrow Island. Winter south-easterly and easterly trade winds and afternoon sea breezes make the open water choppy; mornings are calmest. Tides are moderate (around 2 m) but currents run around the reefs.

Visibility typically 5–15 m. Cyclone season November–April — the Onslow coast is highly cyclone-prone. Stingers possible in the warmer months.

Descend to 14 m and explore the ledges and caves with a torch, then ascend.

No resident dive operator – divers bring their own gear and boat (boat hire available at the resort); scuba gear hire via Pilbara Dive and Tours, Karratha, with advance notice.