Trimix Corner is the starting point for the deepest dive in Victoria. The Scuba Doctor listing gives the site a range of 30–103 m and describes it as the corner of the Nepean-side wall from which divers 'descend along the wall seeking the deepest spot' – Entrance Deep, 'Victoria's deepest known dive location at 103 metres, the bottom of the ancient River Yarra just inside the Port Phillip Heads in the main shipping channel'. It lies 28 m from the Nepean Wall mark; the recreational Nepean Wall dive and this one begin within a boat-length of each other and have nothing else in common.

The wall below 40 m is, in the listing's words, 'very black, barren and rocky, with incredibly treacherous currents'. The encrusting life of the upper terraces gives way to bare rock and rubble in the dark of the channel floor, and the interest of the dive is the place itself – the deepest point of the drowned river that carved The Rip, in the middle of the shipping lane through which the port's traffic passes. Sevengill sharks are the most likely large animals.

Everything about the dive is dictated by the tide. The slack is very short at the entrance, and the ascent must be made up the shot line for as long as possible, because once the ebb begins it 'will quickly push you back over Rip Bank with a minimum depth of 20 metres' – that is, a team on decompression will be carried out through the Heads, and the recovery happens outside in Bass Strait. That requires a dedicated surface team, agreement with Lonsdale VTS, calm seas and divers whose deco can be completed drifting in the open. It is a dive for a handful of the state's most experienced trimix divers, made rarely, and it is included here because it is the extreme end of the Port Phillip Heads system rather than because anyone should plan to do it.

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 on the Nepean side of the entrance beside Nepean Wall, in the main shipping channel; the surface support boat must hold a shot line, track the team throughout and coordinate closely with Lonsdale VTS because deco is completed in the channel. 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 flood (SWF), so that any current after the turn carries divers back into the bay rather than out through The Rip. 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's guidance is uncompromising: 'slack water is very short, and the ascent must be back up the shot for as long as possible as the outgoing tide will quickly push you back over Rip Bank with a minimum depth of 20 metres'; the ascent must begin while the incoming tide continues, and recovery may occur outside the Heads. Flat seas at the entrance are essential.

Trimix team only. Descend the shot at the very start of the flood slack, follow the wall down from the 30 m corner to the target depth (60–80 m for most teams; 103 m at Entrance Deep for the very experienced), turn after a short bottom time, and begin the ascent up the shot before the slack ends; complete decompression drifting under a DSMB with the boat tracking, accepting that the ebb may carry the team back over Rip Bank and out of the Heads.

Trimix with multiple deco gases and full team/surface support; bottom times of minutes; CCR strongly preferred; decompression drifting under DSMB with tracking boat, possible recovery outside the Heads; only on flat seas at the flood slack.

No recreational operator. Dived only by experienced trimix teams from private and technical-club boats, occasionally supported by Dive Victoria Group or Red Boats as a dedicated technical charter. Entrance Deep at 103 m is Victoria's deepest known dive.