The Chalmers was a 595-ton, 40 m three-masted timber barque sailing from Mauritius with sugar and 300 tons of ballast stones. On 19 March 1874 Captain W. R. Alexander, approaching what he believed was the safe passage into Fremantle north of Rottnest, mistook a bushfire on the mainland for the Rottnest lighthouse. The ship struck the reef once, but, confident of his position, he sailed on. After a second hard impact a third holed the hull. The captain's certificate was later cancelled indefinitely for negligence.

The wreck lies in about 3 m on the Sisters Reef in Warnbro Sound. After 150 years little structure remains. The main feature is a large kelp-covered mound marking the ballast pile, with ship's timbers, brass fittings and ballast stones scattered around it. Finding them takes patience and a slow search among the kelp, which is heavy in summer. The site is protected as a historic wreck.

The reef supports buffalo bream, wrasse, sweep and herring cale, and sea lions from the Shoalwater islands may visit.

The Chalmers suits snorkellers and divers interested in maritime history who can dive it together with the Sisters reef on a calm day. It is not a structural wreck dive, and divers expecting a hull will be disappointed. Its story, a ship lost because a bushfire was taken for a lighthouse, is a reminder of how dangerous this coast was before modern navigation aids. The WA Museum holds further records.

Because the site is so shallow, bottom time is limited only by air and warmth, and a slow, methodical search pattern around the ballast pile gives the best chance of spotting fittings. A torch helps to pick out brass and timber in the shade of the kelp.

Launch at the Bent Street ramp in Safety Bay or at Port Kennedy; the run to the Sisters Reef takes less than 10 minutes. The wreck lies at S 32°22.030' E 115°41.437'. Anchor in sand beside the reef, never on it.

The Sisters reef dive is on the same reef system, so both can be done from one anchorage in calm weather.

Needs swell under about 1.5 m, and winds over 15 knots make access difficult. Easterly winds give the best visibility; westerly swell and the summer south-westerly sea breeze create surge on the shallow reef. Visibility falls after winter storms.

North-westerly winds ahead of winter fronts make Warnbro Sound rough, so this is mainly a summer and autumn site.

Anchor in sand nearby, find the large kelp-covered mound of the ballast pile and search slowly around it for timbers, fittings and ballast stones in 2–4 m. Snorkelling works as well as scuba here.

No regular operator; private boats and clubs.