Challenger Rock is a reef rising to within a metre of the surface at low tide, just south-east of the leading-light buoys near the northern tip of Garden Island. The Marine Life Network describes it as a fairly sheltered dive about 10 minutes from the Rockingham ramp, with a maximum depth of about 14 m, great fish life around the rock and caves on its eastern and central sections.

It is also a site of historical interest. The Sepia, an iron barque lost in 1898, is recorded by the Marine Life Network as having hit Challenger Rock, and her wreck lies nearby between Carnac and Garden Islands.

The reef is limestone covered in kelp and sponges. The caves and overhangs give shelter to buffalo bream, bullseyes, sweep and morwong, with western rock lobster and the occasional wobbegong inside, and wrasse and leatherjackets on the reef top. Average visibility is about 6 m.

Its sheltered position makes it a good alternative when the outer bank and Rottnest are rough, and the short run from the ramps suits small boats. It suits Open Water divers, and the shallow top makes a good safety-stop area.

This record relies on a single source, and the position given is approximate. Garden Island is Defence land, with restricted waters and landing rules, so check current restrictions before planning any dive in the area. Divers who know it combine it with the Sepia for a two-dive morning, finishing on the shallow top of the rock, which also makes a convenient place for a safety stop.

Launch at the Rockingham or Woodman Point boat ramps; the Marine Life Network puts it about 10 minutes from the Rockingham ramp (Woodman Point is a similar distance). Head for the leading-light buoys off northern Garden Island; the rock lies just to their south-east. The site is fairly sheltered.

Anchor in sand clear of the reef and keep well away from the Defence restricted areas around Garden Island.

The Marine Life Network describes the site as fairly sheltered, protected by Garden Island and the northern end of Five Fathom Bank. Strong westerly and north-westerly winds still produce chop and surge over the shallow rock. Dive in the morning before the summer sea breeze.

Average visibility is about 6 m.

Anchor in sand near the rock, descend to 10–14 m and work around the reef, exploring the caves on the eastern and central sections, then finish on the shallower parts of the rock.

No regular operator; private boats and clubs.