The Cave is the largest single cavern on the Lonsdale Wall system, a chamber at 30 m that the Scuba Doctor listing describes as 'big enough to drive a small truck into'. It sits at the very ocean end of the wall on the western side of the entrance, 35 m from Foggy Reef, where the cliff runs east–west with the deep water of the Rip channel to the south and the base at about 60 m.

The cavern is a true room in the rock rather than a swim-through: a wide mouth in the face at 30 m opening into a chamber with a rock roof and a rubble floor, and enough space for several divers to enter together. It is not a penetration in the cave-diving sense – daylight reaches well inside – but lights are needed to see the walls, which are covered in the zoanthids, jewel anemones and sponges that grow wherever the Heads current reaches. Southern blue devils and southern rock lobster live in the back of the chamber, and the shelter draws bullseyes and old wives. Outside, the face continues down in the usual steps, and technical divers use the base for deep dives.

The position sets the rules. The Cave is in the shipping channel and at the point where the ebb runs hardest out of the bay, so the listing restricts diving to slack water at the end of the ebb with no shipping due – a narrower set of conditions than the central wall, and one reason it is dived less often. With the top of the wall at 22 m there is no shallow water at all, so the whole dive is at depth and short. For experienced deep divers who catch it on a calm day it is one of the most impressive single features at the Heads.

Boat access only. Charter boats leave from Portsea Pier (Dive Victoria Group, Bayplay) and Queenscliff Harbour (Dive Victoria Group, Red Boats); run time is about 15 minutes. Private boats launch at Sorrento Boat Ramp (Point Nepean Road, Sorrento, all-tide concrete ramp with trailer parking) or Queenscliff Boat Harbour ramp (Larkin Parade, Queenscliff), roughly 15 minutes to the site.

The site is at the ocean end of Lonsdale Wall in the channel, next to Foggy Reef. Skippers must call Lonsdale VTS (Point Lonsdale) or listen to VHF 12 for shipping movements before dropping divers, and must carry a dive flag; anchoring is not permitted in the shipping channel, so a live boat and a drop-shot or drift float is the normal method.

It is dived only on slack water at the end of the ebb (SWE); on other tides the current across the site is unworkable. Slack water at Port Phillip Heads does not coincide with high or low water: it falls roughly three hours after high or low water at Point Lonsdale, and the usable window is typically 20–40 minutes. Use the Ports Victoria / Port of Melbourne slack-water predictions for The Heads (also republished by charter operators and WillyWeather 'Rip Bank') rather than a standard tide table.

Being outside or right at the entrance, it is exposed to Bass Strait swell: anything over about 1.5 m from the south-west or south, or a strong southerly/westerly wind against the ebb, produces standing waves in The Rip and charters cancel. Northerlies flatten the entrance and give the best days. Visibility is best in late summer and autumn (Feb–May) when the water is clear and the westerlies have eased; winter brings 11–13 °C water and bigger swells but often the clearest days after a run of settled weather.

Heavy rain has little effect this close to the ocean. The listing states it 'should only be dived when there is no shipping and only at slack water at the end of an ebb or outgoing tide'.

Shot onto the wall top at 22 m at the ebb slack. Drop straight to the cave mouth at 30 m, enter with lights (the chamber is large and open, no line needed), spend a few minutes inside, then work the adjacent overhangs at 28–32 m before climbing back to the reef top. DSMB from 22 m; the boat must be tracking, as the ebb will resume and set seaward.

Wall base ~60 m; ebb-slack only, in the channel. Trimix for the base; all deco drifting under DSMB with tracking boat. The cavern itself is open and daylight-lit – no line or cave certification required, but lights are.

Dive Victoria Group (Portsea/Queenscliff) – runs the deep Heads walls for advanced and technical divers on suitable slack-water days; Red Boats (Queenscliff) – deep-wall and technical charters on request; otherwise private club boats (VSAG, Power House Underwater Club, Scuba Diving Melbourne). Not a routine training-boat site.