An exact number of atolls worldwide varies depending on source, but a census<\/a> published in 2023 yielded 494 oceanic atolls and 104 continental atoll systems. 344 were located in the Pacific Ocean (including Japan), 77 in Indonesian waters, 68 in the China Sea, 54 in the Indian Ocean, and 29 in Australia’s Coral Sea and the Pacific and Indian ocean waters surrounding the continent. <\/p>\n The word \u2018atoll\u2019 derives from the Maldivian word \u2018atholhu\u2019, which can be understood as \u2018circular group of coral islets.\u2019\u00a0<\/p>\n How do atolls form?<\/p>\n It was Charles Darwin<\/a> who first proposed a theory on atoll formation in 1842 \u2013 this theory is called the subsidence model. According to it, atolls develop from underwater volcanoes<\/a>, called seamounts. First, the volcano erupts and lava piles up on the seafloor. The elevation grows higher and higher, eventually emerging above the surface of the water and becoming an island.\u00a0<\/p>\n In the next stage, corals build a reef around the island \u2013 a fringing reef forms, surrounding the island just below the surface. The strip of water between the reef and the island is the lagoon.<\/p>\n Over time, the volcanic island erodes and sinks, dissipating into the sea in a process called subsidence. The top of the reef keeps growing upwards at roughly the same rate that the island sinks, so it stays close to the surface even as the land beneath disappears. Eventually, the volcanic peak vanishes, leaving only the ring of coral, which has now become a barrier reef.\u00a0<\/p>\n The atoll is now fully formed \u2013 this entire process can take between 1 and 30 million years.<\/p>\n<\/p>\n However, this model has been challenged and an alternative theory, first proposed in the 1930s, is the antecedent karst model<\/a>.<\/p>\n In this model, a limestone island built from an old, long-dead reef sits exposed above the sea during a period of low sea levels. Rainwater slowly dissolves the limestone through a process called karstification, forming a hollowed-out basin in the middle of the island. When sea levels rise, coral colonises the raised outer rim of this basin, and the centre floods to become the lagoon.\u00a0<\/p>\n What is the largest atoll?\u00a0 The Great Chagos Bank, located in the Indian Ocean roughly 500km south of the Maldives, stretches over a massive 12,642 square kilometres. Most of it is located underwater; only a handful of small islets break the surface, with a combined land area of just 5.6 square kilometres. Administered by the UK as part of the British Indian Ocean Territory, it\u2019s also a protected Strict Nature Reserve.<\/p>\n Kwajalein Atoll \u2013 largest lagoon<\/p>\n Located in the Marshall Islands in the Pacific Ocean, Kwajalein is made up of around 97 islands and islets ringing a lagoon often cited as the largest in the world at roughly 2,174 square kilometres \u2013 though its land area is just 16.4 square kilometres.\u00a0<\/p>\n Kiritimati, also known as Christmas Island, has the biggest dry land area of any atoll on the planet at 321 square kilometres.\u00a0This atoll is part of Kiribati, an island nation in the Pacific Ocean.<\/p>\n This atoll lays in the southern Maldives, in the Indian Ocean, and wins on sheer number of islands: 255 in total, spread across a total area of 3,152 square kilometres.\u00a0<\/p>\n How many people live on atolls?<\/p>\n About one million people inhabit coral atoll islands worldwide, with roughly half of this population residing across major atoll-based island nations and territories, such as the Maldives, Kiribati, Marshall Islands and Tuvalu.<\/p>\n What wildlife lives on atolls?<\/p>\n Land fauna on atolls is sparse due to small land areas, but they still support a rich variety of life, from thriving marine ecosystems in their shallow lagoons to critical breeding grounds for seabirds on their sandy islets.<\/p>\n According to a study<\/a> published in the journal Nature Ecology & Evolution, there are 37 species<\/a> of seabirds that breed on atolls – and the populations of each range from a few dozen to over 3 million individuals per atoll. The study estimates that, in total, over 31 million seabirds nest on atolls – about one quarter of all tropical seabirds in the world. <\/p>\n Atolls are critical for the survival of species such as white terns<\/a>, black noddies and lesser frigatebirds<\/a>, since over half of their global populations nest there.<\/p>\n Are atolls at risk due to climate change?\u00a0<\/p>\n
\nGreat Chagos Bank \u2013 largest total area<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2026\/08\/960px-Kwajalein_Atoll_2003-02-07_-_Landsat_7_-_30m.png\")
Satellite image of Kwajalein Atoll, Republic of Marshall Islands. Credit: NASA, Public domain, via Wikimedia Commons
\nKiritimati Atoll \u2013 largest land area<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2026\/08\/Kiritimati-EO.jpg\")
Kiritimati Island\u00a0as seen from the\u00a0International Space Station.\u00a0Credit: NASA, Public domain, via Wikimedia Commons
\nHuvadhu Atoll \u2013 most individual islands<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2026\/08\/GettyImages-1255958571-scaled.jpg\")
White tern (Gygis alba), Midway Island, Midway Atoll, Hawaiian Islands. Credit: Gerald Corsi\/Getty Images
\n![\"\"](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2026\/08\/GettyImages-2240355441-scaled.jpg\")
White-capped noddy (Anous minutus). Credit: BrianScantlebury\/Getty Images
\n![\"\"](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2026\/08\/960px-Lesser_Frigatebird_from_Indonesia.jpg\")
Lesser frigatebird\u00a0(Fregata ariel). Credit: Shah Jahan, CC BY-SA 4.0, via Wikimedia Commons<\/p>\n