An atoll is a ring-shaped coral reef, usually topped by low sandy islets, that encloses a lagoon with no central island. Most atolls start as a fringing reef around a volcanic island: as the volcano slowly sinks and erodes, the coral keeps growing upward toward the light, until only the reef ring and its lagoon are left above the old summit. There are roughly 440 atolls in the world, most of them in the Pacific and Indian oceans. The largest atoll structure is the Great Chagos Bank in the Indian Ocean, at 12,642 km², while Kiritimati in Kiribati has the most dry land of any atoll that encloses a lagoon, about 312 km². The word itself comes from the Dhivehi language of the Maldives, where atholhu means this kind of reef ring.
Darwin’s Subsidence Theory, Sketched Aboard HMS Beagle
The explanation still taught today began with Charles Darwin. He worked out the idea during the voyage of HMS Beagle, while still in South America and before he had seen a single coral island, and wrote it up as the ship crossed the Pacific, finishing his draft by November 1835. Captain Robert FitzRoy’s Admiralty orders included surveying an atoll, and on 1 April 1836 the crew began work at the Keeling (Cocos) Islands in the Indian Ocean, taking soundings around the reef and inside the lagoon. Darwin published the result in May 1842 as The Structure and Distribution of Coral Reefs, his first monograph.
His argument was a sequence in three steps. First, corals build a fringing reef hugging the shore of a tropical volcanic island. Second, as the island subsides, the outer edge of the reef keeps pace with sea level because that is where waves bring the most food and oxygen, while the water behind it deepens into a lagoon; the reef is now a barrier reef, separated from the shrinking island. Third, the volcano disappears below the surface altogether and the ring of reef remains: an atoll. The lagoon is not a drowned crater; it is the space the island used to occupy.
Darwin pointed to real islands at each stage. Bora Bora in the Society Islands of French Polynesia, where the eroded peaks of Mount Otemanu and Mount Pahia rise to 727 m inside a wide lagoon and a ring of reef islets, is the textbook “almost atoll”. Aitutaki in the Cook Islands is another. Roughly 230 km to the east, atolls such as Tetiaroa, 53 km north of Tahiti, show the final stage, with no volcanic rock visible at all.
Drilling for Proof at Funafuti (1896–98) and Enewetak (1952)
Darwin’s theory was not universally accepted. In the late 19th century the Scottish oceanographer John Murray, a veteran of the 1872–76 Challenger expedition, argued that reefs grew on mounds of marine sediment that had built up to shallow depths, with no sinking required. The wealthy American naturalist Alexander Agassiz financed expedition after expedition to find evidence for Murray and against Darwin.
The obvious test was to drill. If Darwin was right, a bore through an atoll should pass through a great thickness of shallow-water coral before hitting volcanic rock far below the depth at which corals can live. The Royal Society of London sent three expeditions to Funafuti, now the capital atoll of Tuvalu, in 1896, 1897 and 1898. William Johnson Sollas of Oxford led the first, Edgeworth David of the University of Sydney the second. The bores reached a final depth of about 340 m without striking volcanic rock, and at the time the result was regarded as inconclusive.
The answer came from the United States Atomic Energy Commission. Before the nuclear tests at Enewetak Atoll in the Marshall Islands, test bores in 1952 went down through roughly 1,400 m of coral limestone before striking the basalt of the underlying volcano. Since reef-building corals live only in the top 100 m or so of the ocean, that coral could only have accumulated on a foundation that had been sinking for tens of millions of years. Enewetak’s seamount, formed in the late Cretaceous, now lies about 1,400 m below sea level. Darwin had guessed a maximum coral thickness of around 1,500 m; he was remarkably close.
The Antecedent Karst Model and Ice-Age Sea Levels
Subsidence explains why atolls sit on sunken volcanoes, but later geologists argued that it does not fully explain the ring shape. In the antecedent karst model, developed by J. E. Hoffmeister, F. S. McNeil and others, the key is the swing in sea level during the Pleistocene ice ages. When sea level fell during glacial periods, flat-topped reef platforms were exposed to the air. Rainwater dissolved the limestone, and because dissolution was slowest at the edges and fastest in the middle, each platform weathered into a saucer with a raised rim. When interglacial seas flooded it again, coral recolonised the rim and the hollow centre became the lagoon.
Research by A. W. Droxler, Stéphan Jorry and colleagues supports this view for many modern atolls. Their work found that the reefs buried beneath the atolls they studied were flat-topped platforms, not rings, and that the atoll shape itself appeared only after Marine Isotope Stage 11, around 400,000 years ago. The two models are not entirely rivals: subsidence supplies the sinking foundation and the thick limestone cap, and ice-age sea levels help sculpt the ring on top.
Kure Atoll and the Darwin Point
An atoll can only survive where coral grows fast enough to match the sinking of its base. Reef-building corals need warm, clear, sunlit water, so atolls are confined to the tropics and subtropics. The Hawaiian chain shows the limit clearly. The islands are carried northwest by the Pacific Plate away from the Hawaiian hotspot, sinking as they age. Near the end of the chain lies Kure Atoll, at 28°25′ N the northernmost atoll in the world: a coral ring about 9.7 km across around a lagoon several metres deep. Beyond it, the older volcanoes of the chain have drowned as seamounts because the water is too cool for coral to keep up. Geologists call this threshold the Darwin Point.
At the other end of the range, Elizabeth Reef at 29°57′ S in the Tasman Sea is the southernmost atoll, and Aranuka in Kiribati, whose southern tip lies just 13 km north of the Equator, is the closest to it. Bermuda, at 32°18′ N, is sometimes called an atoll, but its high ground and mostly submerged reefs lead geographers to class it as a pseudo-atoll kept alive by the warm Gulf Stream.
Lagoon Giants: Great Chagos Bank, Huvadhu and Kwajalein
Measured by the area inside the reef, the biggest atolls are mostly water. The Great Chagos Bank, about 500 km south of the Maldives, covers 12,642 km², yet its eight low sandy islands add up to only about 5.6 km² of land; most of its rim is wholly submerged. All its islands have been a Strict Nature Reserve since 1998.
Huvadhu Atoll in the southern Maldives covers 3,152 km², of which just 38.5 km² is dry land, and holds the record for the most islands of any atoll, with 255. It has been split into two administrative atolls, Gaafu Alifu and Gaafu Dhaalu, since the 1970s. In the Marshall Islands, Kwajalein Atoll strings 97 islands and islets, with 16.4 km² of land, around one of the largest lagoons on Earth, 2,174 km². Its islands average only about 1.8 m above sea level.
Raised Atolls: Kiritimati and Aldabra
Not every atoll stays at sea level. Where the seabed has been pushed up, or sea level has fallen, the old reef can stand clear of the water as a raised atoll, with limestone cliffs and far more land. Kiritimati, or Christmas Island, in the northern Line Islands of Kiribati, has about 312 km² of land and a 328 km² network of lagoons; it alone makes up more than 70% of Kiribati’s land area. It lies 232 km north of the Equator and in the UTC+14 time zone, among the first inhabited places to see each new year.
Aldabra in the Outer Islands of the Seychelles, 34 km long and 13 km wide, rises to about 8 m and encloses a shallow lagoon of 196 km² that is roughly two-thirds dry at low tide. Because it has no surface fresh water, it was never heavily settled, and it remains home to the Aldabra giant tortoise. It became a UNESCO World Heritage Site on 19 November 1982. Raised coral islands that are sometimes listed alongside these, such as Lifou in New Caledonia (1,146 km²) and Rennell Island in the Solomon Islands (660 km²), are larger still, but sources disagree on whether they count as atolls.
Countries Built on Atolls: Maldives, Tuvalu and the Marshall Islands
Ordinary atoll islands are made of coral sand and rubble thrown up by waves, and they rarely rise more than 5 m above the sea. That makes the few countries built almost entirely on atolls the lowest on Earth. The Maldives has 1,192 coral islands in 26 atolls stretching 871 km north to south, with only 298 km² of land in roughly 90,000 km² of ocean; its highest natural ground is about 2.4 m and its average about 1.5 m. Tuvalu, with six true atolls and three reef islands, peaks at 4.6 m on Niulakita. The Marshall Islands are made up of 29 coral atolls and five single islands in the Ratak and Ralik chains.
Atoll islands are not static. A 2018 study of Tuvalu’s nine atolls and 101 reef islands found that between 1971 and 2014 about three-quarters had grown in area as waves moved sand onto their shores, a net gain of more than 2%. Tuvalu’s then prime minister, Enele Sopoaga, replied that the country was not expanding and had gained no additional habitable land, and Tuvaluan leaders have pointed to saltwater intrusion into groundwater as a pressing threat. When Cyclone Bebe swept over Funafuti in October 1972, it submerged the atoll and destroyed 95% of the structures on the island.
From Darwin’s soundings at the Keeling Islands to the Enewetak drill cores, the story of the atoll is the story of a volcano disappearing while a living reef refuses to drown. To see how the world’s biggest examples compare, read our ranking of the largest atolls in the world; for how atolls fit beside fringing and barrier reefs, see the types of coral reefs; and for the atoll nation that gave the landform its name, explore the geography of the Maldives.