Bennetts Bottom is one of two deep natural reefs listed by marinelife.org.au for Darwin, a very attractive reef at about 32 m suitable only for experienced divers, home to many kinds of fish and corals. Beyond that, it is poorly documented, and its exact location is local knowledge. 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.

Deep reefs in the Darwin area are less affected by the surface murk and wave action than the shallow harbour sites, and tend to carry larger gorgonians, sponges and soft corals, with bigger fish. At 32 m, however, a Darwin reef dive combines depth, darkness and the ever-present risk of the current returning, so it is not a site to take lightly. It suits divers with deep certification, good gas planning and experience of the local tides, diving with someone who knows the site. Divers who do visit could help by recording depth, layout and species, since published information is thin. If Bennetts Bottom proves hard to arrange, The Ridge offers a similar deep-reef experience, and the Fenton Patches wrecks offer deep diving in clearer water. Ask DSAC whether the site is on the club's current program before planning a trip.

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 quickly to the reef at about 30 m, explore close to the line, and begin the ascent with plenty of gas in hand - no-deco time on air is very short. Safety stop on the line.

Nitrox 28% or less is the practical maximum at 32 m (MOD). Carry a DSMB and two torches.

No regular operator; dived by experienced local divers and occasional DSAC outings with the mark.