The Miwok II is an obscure but interesting site off Bickley Point. She was a 33 m steel crane barge of 237 tons that had worked for Hamersley Iron at Dampier before being brought south. In 1983, at the WA Museum's request for an alternative to the RMS Orizaba (a historic wreck then being used for military training), she was scuttled off Rottnest and used by the Special Air Service Regiment for underwater demolition training.

The result is a battered steel barge lying in about 11 m on sand, showing the effects of both explosives and four decades of swell. Divingwawrecks describes it as a largely unknown site, so there is little published detail of its present condition; expect a low profile with broken plating and crane remains that attract reef fish, and clear water of 10-15 m typical of the south-east side of the island.

It is a short, simple dive best suited to divers with their own boat who want something different from the busy Thomson Bay wrecks, or who are exploring the Porpoise Bay and Dyer Island area, where the Lady Elizabeth and Crystal Palace are close by. Because it is less than 75 years old it is not a protected historic wreck, but it lies within the Rottnest Island Reserve, where spearfishing is banned. Treat the site as a navigation challenge as much as a wreck dive: with little published information, finding and mapping it is part of the appeal, and a well-marked GPS and a good sounder are essential.

Launch at Fremantle, Woodman Point or Hillarys and run about 18-20 km (30-45 minutes in a trailer boat) to the south-east corner of Rottnest, rounding Bickley Point from Thomson Bay. Locate the barge with an echo sounder using the divingwawrecks GPS and anchor in sand clear of the wreck. There is no regular charter to this site.

Rottnest lies about 18 km west of Fremantle; allow 45-90 minutes each way by charter or trailer boat depending on the site. Follow Rottnest Island Authority mooring and anchoring rules - do not anchor on reef or in sanctuary zones - carry a marine radio and log a trip plan, as the crossing of Gage Roads can turn rough when the afternoon sea breeze arrives.

The south-east corner of Rottnest is sheltered from westerly swell but open to southerly and south-easterly winds; divingwawrecks gives a working limit of under 2 m swell. Summer easterly mornings are ideal. Avoid strong south-westerly sea breezes, which build a short chop across Bickley Bay by early afternoon.

General pattern at Rottnest: summer brings calm morning easterlies followed by a strong south-westerly sea breeze from late morning, so dive early; winter brings north-westerly fronts and large swell, with clear, calm spells between them. Check the Bureau of Meteorology coastal forecast for swell height and period before committing.

Drop the anchor on sand beside the barge, descend and circuit the hull at 10-11 m, then rise to the deck and crane remains; ascend up the anchor line. Agree a turn pressure before descending (half gas, or a third where current is likely), carry a DSMB, and make a 3-minute safety stop at 5 m.

No regular operator - private boat and dive club dives only; the site is described by divingwawrecks as largely unknown to the diving community.