Deep Pinnacles is the deeper continuation of the outer Lonsdale reef, a field of 'scattered reef and rock bommies rising out of the sandy sea bed' at 16–25 m south-west of Point Lonsdale, in line with the Lonsdale Arches reef and 250 m from Castle Rock. The Scuba Doctor listing (GPS verified) describes the soft calcarenite as 'eroded into lots of little swim-throughs, valleys and overhangs, all topped off with a light scattering of kelp', and the colour as 'yellow zoanthids and awesome colour'. Golden Arch, a natural arch to 23 m, is 137 m to the west on the same line.

The bommies stand a few metres proud of the sand, separated by valleys and linked by short tunnels that the current has bored through the soft rock; the undersides of every overhang are yellow with zoanthids and studded with sponges and jewel anemones, and the kelp on top is thin because of the depth. The listing's fauna – crayfish, blue devils, abalone 'and much more' – is that of the whole outer reef, with the addition of the animals that prefer deeper water: bigger schools of sweep and boarfish, Port Jackson sharks under the ledges, cuttlefish in season, and the occasional sevengill shark or seal passing over the sand.

Because it lies a little further from the entrance than the Arches and Castle Rock the tide is gentler and the listing rates it diveable on any tide as well as at slack; the constraint is Bass Strait swell, and the boat ride out through the Heads. At 16–25 m it is an Advanced site and a natural step up from Castle Rock for divers who have done the shallower outer reef; it is dived by charters on calm 'outside' days and by club boats, and it rewards a torch and a camera.

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 22 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 22 minutes to the site.

The site is outside the Heads south-west of Point Lonsdale between Castle Rock and Golden Arch, about 20–25 minutes from Queenscliff and 25–30 from Portsea on a calm day. 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 can be dived on most tides, but slack water gives the easiest conditions and the best visibility. 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. Scuba Doctor lists it for slack, ebb and flood – being further from the entrance the tide is weaker here and the limit is swell: any south-westerly sea above about 1.5 m rules it out, and light northerlies give the best days.

Shot onto a bommie top at 16 m. Work from bommie to bommie across the sand at 20–25 m, threading the small swim-throughs and valleys and lighting the overhangs for zoanthid colour and crayfish; navigate back to the shot by compass and the reef line, and ascend it or a DSMB.

Dive Victoria Group (Portsea and Queenscliff) – double boat dives to the Heads walls most days of the week, site chosen on the morning by slack-water time and swell; Bayplay Adventure Tours (Portsea) – weekend and holiday charters; Red Boats (Queenscliff) – weekend Heads charters; Melbourne clubs (VSAG, Power House Underwater Club, Scuba Diving Melbourne) run their own boats here regularly.