The Mauna Loa was a 5,436-ton American cargo ship built by the Los Angeles Shipbuilding and Dry Dock Company at San Pedro in 1919: a 125 m, two-deck, two-masted steamer with an oil-fired triple-expansion engine. Like the Meigs she was in Darwin with the convoy bound for Timor. While at anchor on 19 February 1942 she was hit by two bombs that went down an open hatch, starting fires and breaking her back, and she sank quickly.

Sources differ on casualties: DSAC records five dead from a crew of 38, while other accounts say none of her crew or passengers was injured. The 19 February 1942 raids - two waves of Japanese carrier and land-based aircraft, the first and heaviest attack ever made on mainland Australia - sank eight ships in the harbour and killed more than 230 people. In 1959-60 the Japanese firm Fujita Salvage cut most of the wrecks down to or below the waterline for scrap, so what divers find today is usually the lower hull, machinery that was too heavy to lift, and the cargo that the salvors were not permitted to remove.

Mauna Loa was heavily salvaged in 1960 and today lies about 18 m down as a twisted, broken lower hull on a sand and silt bottom. The salvors were barred from taking the American cargo, and the holds still contain the war materiel she was carrying: military trucks, Bren gun carriers, a Harley-Davidson motorcycle and large quantities of .303 and .50-calibre ammunition. Picking out recognisable pieces in the gloom is the appeal of the dive.

It is often combined with the Meigs on the same neap-tide window, and the pair give the clearest picture of what the Timor convoy was carrying. The 2015 multibeam survey of the harbour produced detailed images of the site showing its twisted deck littered with ammunition, motorcycles and gun carriers. Suitable for Open Water divers with some low-visibility experience; a good torch makes a big difference.

Boat only. Darwin's charter operators leave from Cullen Bay Marina or the Darwin Waterfront precinct and run across the harbour to the site in well under half an hour on a calm neap-tide morning. Private boats can launch at the Cullen Bay public ramp (west of the CBD off Marina Boulevard), the Stokes Hill / Frances Bay ramp beside the Waterfront, or the East Arm ramp for the southern anchorage.

Mark the position on GPS before the dive - the wrecks lie in open water with no surface marker - and drop a shot rather than anchoring into protected structure. Parking and trailer bays are at all three ramps; air fills and hire through the Darwin dive shops.

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 shot at neap slack and work the former holds, where the ammunition and motorcycle remains lie. Stay at 17-19 m just above the silt, keep the shot line direction in mind, and return along the hull remains for the ascent and a safety stop on the line.

Darwin Dive Centre and Dive Darwin run harbour wreck charters only on neap-tide windows (typically a few days either side of each neap, weekends favoured); Darwin Sub Aqua Club dives it on club neap-tide days (members plan dives at Thursday meetings at the Darwin Ski Club). No daily schedule.