The Ridge is the deepest natural reef listed for Darwin by marinelife.org.au: a rock wall rising to about 33 m from 40 m, suited only to experienced divers. It carries soft corals, sponges and huge gorgonian corals, with schools of sweetlip and trevally. Darwin's natural reefs are not the clear-water coral gardens of the Great Barrier Reef. The harbour is a macro-tidal, turbid system, and its reefs are low rock platforms and bommies covered in sponges, soft corals, gorgonians, sea whips and hardy hard corals that tolerate silt and big tidal flows. What they lack in visibility they make up for in density of life and the chance of turtles, rays and big fish.

At this depth the ridge is out of reach of the surface swell and much of the silt, so the gorgonians grow large, and schooling fish hang along the face. The dive is short on air and dark, and the time you have is best spent close to the face looking at the fans and the fish rather than roaming. The combination of depth, low light and the potential for the tide to begin running makes it an expert site, best done with a technical approach - appropriate gases, planned decompression and a boat that can follow a drifting DSMB. Published information is limited to a short description, and the site's position is local knowledge, so plan it with local divers who have been there. For the right diver, it offers a side of Darwin diving that few people see.

Boat only. Launch at Cullen Bay Marina or the Cullen Bay public ramp (off Marina Boulevard, 5 minutes from the CBD, trailer parking) and run out into the harbour; most harbour reefs are 10-25 minutes away on a calm neap morning. Positions are known to local operators and DSAC rather than widely published, so go with someone who has the mark or use a good chart and sounder.

Anchor on sand beside the reef, not on coral, and plan to arrive before high-water slack.

Tide is everything in Darwin Harbour: the spring range is 7-8 m and tidal streams run hard through the harbour, so diving is generally only possible on neap tides (range under about 3-4 m), timed to the slack at high water when clearer ocean water sits over the site. Spring tides bring strong currents and silt-laden water with near-zero visibility. The dry season (May-October) gives the most consistent conditions; the south-east trade winds can chop up exposed water in the afternoon, so mornings are best.

In the wet season (November-April) run-off from the rivers and creeks drops visibility, afternoon storms build quickly, and the harbour is in cyclone season; box jellyfish are present October-May.

Descend the anchor line to the top of the wall at 33 m and follow it briefly, keeping above 40 m. On air, bottom time is very short; technical divers plan staged decompression. Ascend on the line or a DSMB.

Trimix, or air with planned decompression; air (21%) is the richest practical bottom mix at 40 m. Carry DSMB, reel and backup lights. Live-boat pickup.

No regular operator; experienced and technical local divers only, with the mark.