The islands of the archipelago have a collective land area of about 22 km2. The largest islands, Hermite and Trimouille have areas of 1022 ha and 522 ha respectively. They consist of limestone rock and sand. The rocky parts are dominated by Triodia hummock grassland with scattered shrubs, while the sandy areas support grasses, sedges and shrubs, mainly Acacia. Patches of mangroves grow in sheltered bays and channels of the archipelago, especially at Hermite Island. The climate is hot and arid with an annual average rainfall of about 320 mm.
Wildlife
Birds
The islands have been identified by BirdLife International as an Important Bird Area because they support over 1% of the world populations of fairy and roseate terns, and of sooty oystercatchers. Greater crested terns breed there irregularly, sometimes in large numbers. Other birds breeding on the islands include ospreys, white-bellied sea eagles, pied oystercatchers, Caspian terns and bridled terns. The islands support 12–15 breeding pairs of beach stone-curlews. Yellow white-eyes have been recorded.
It is believed that the islands were occupied by indigenous Australians until about 5,000 BC, when a period of global warming and rising sea levels caused the Montebellos to become more distant from the mainland, and forced abandonment. The first Europeans known to have seen the islands were the crews of a French Navy exploration expedition, led by Nicolas Baudin, in 1801. Baudin named Hermite Island after Admiral Jean-Marthe-Adrien L'Hermite and Trimouille Island after a French aristocratic family. There are inconsistencies in the naming of the islands in the accounts of the early explorers, and it has been suggested that later explorers mixed up the names of Lowendal and Hermite Islands. An early reference to the islands is in 1622, when the English ship Tryall was wrecked just west of them. For years afterwards their approximate position was recorded on charts as the Tryal Rocks. The islands were economically significant for pearl fishing from the end of the 19th century until the outbreak of the Second World War.
The Montebello islands were the site of three nuclear weapons tests by the British military: one in 1952, and two in 1956. A bay on Trimouille Island was the site of Operation Hurricane, the first-ever atomic weapon tested by the United Kingdom, on 3 October 1952. While most subsequent British tests were conducted at sites on mainland Australia, in 1956 there were two further tests, on Alpha and Trimouille Islands respectively. The second of these, codenamed "G2", included the largest nuclear explosion in Australia, with a yield of 98 kilotons. Fallout from the Montebello tests is reported to have contaminated areas of mainland Australia as far away as the Queensland towns of Mount Isa, Julia Creek, Longreach and Rockhampton.
