HMS Plym (listed by Image Dive as HMAS Plym, about 1,300 tonnes) was the River-class frigate that carried Britain's first atomic device, detonated inside the ship off Trimouille Island on 3 October 1952 in Operation Hurricane. The vessel was destroyed and little remains beyond fragments on the seabed; Image Dive lists the site at about 12 m.

It is one of the strangest dive sites in Australia — less a wreck than a historic location — and should only be visited with local knowledge and awareness of radiation advice for the adjacent islands.

About the Montebellos: the group of more than 200 small limestone islands and islets lies 120 km west of Dampier and is protected as the Montebello Islands Marine Park, with more than 150 hard coral species, over 450 fish species, 170 echinoderms and 630 molluscs recorded. Flatback, green and hawksbill turtles nest here, and humpback, minke and Bryde's whales, dolphins and dugong frequent the waters. The islands carry a unique history: British atomic tests were conducted on and near Trimouille and Alpha islands in 1952 (Operation Hurricane) and 1956 (Operation Mosaic), and visitors are advised to visit rather than camp on those islands. Pearl farming has operated in the lagoons since the early 1900s.

Planning notes: Advanced Open Water certification is recommended and the site is rated advanced. Expect to spend most of the dive around 12 m, with a maximum of about 12 m. It can only be reached on a multi-day expedition or liveaboard, so it is usually one of several sites dived on the trip and choices are made by the crew on the day. Currents are significant, so timing around the tide is part of the plan. Best season: April–October. Traffic: remote/expedition only.

The Montebellos lie about 120 km west of Dampier, 80 nautical miles from Onslow and 120 from Exmouth. There is no day-trip access for most divers: visits are by liveaboard or charter vessel (Montebello Island Safaris, based in Exmouth, runs multi-day charters and advertises dive liveaboards) or by private ocean-going boat, typically a 6–10 hour passage, anchoring in the islands' sheltered lagoons. Carry full safety and communications gear; there are no facilities.

Diving is best April–October outside cyclone season. The islands have a large tidal range (about 6 m at springs), driving strong currents through the channels; dive at slack water. Visibility is unpredictable — one operator suspended charters in 2022 partly because visibility often struggled to exceed 5 m — while the western outer reefs can be much clearer on calm days.

South-westerly and easterly winds limit which anchorages are usable.

Only with an operator who knows the site: descend to about 12 m and view the remaining fragments and crater area; avoid disturbing sediment.

Montebello Island Safaris (Exmouth) – multi-day charters and dive liveaboard trips; Image Dive suspended its Montebello dive charters in 2022. Otherwise private vessels only.