Atolls are among the world most unusual and fragile landforms developing over millions of years from volcanic islands and coral reefs. These ring shaped islands surround lagoons and are mainly found in warm tropical and subtropical oceans.
According to the traditional theory proposed by Charles Darwin in 1842 an atoll begins as an underwater volcano. As lava builds up, the volcano eventually rises above sea level and forms an island. Coral reefs then develop around the island.
Over time, the volcanic island gradually erodes and sinks beneath the ocean. The surrounding coral continues growing toward the surface, eventually leaving behind a ring shaped reef enclosing a lagoon. This process can take between 1 million and 30 million years.
Scientists have also proposed an alternative explanation called the antecedent karst model, in which limestone islands develop hollowed out basins during periods of low sea levels. When sea levels rise, coral grows around the elevated edges and the flooded centre becomes a lagoon.
There are hundreds of atoll systems worldwide mainly across the Pacific and Indian oceans. The Great Chagos Bank is the largest atoll system by total area, covering about 12,642 square kilometres, although most of it remains underwater. Kwajalein Atoll has a lagoon covering roughly 2,174 square kilometres, while Kiritimati has the largest dry land area of any atoll at around 321 square kilometres.
Around 1 million people live on coral atolls worldwide with major populations in countries and territories including the Maldives, Kiribati, the Marshall Islands and Tuvalu.
Atolls are also important habitats for wildlife. Their shallow lagoons support diverse marine ecosystems, while their small islands provide breeding grounds for seabirds. Studies estimate that more than 31 million seabirds nest on atolls, accounting for around one quarter of tropical seabirds globally.
However, climate change is creating growing risks for these low-lying communities. Most atoll land is only about 1–2 metres above sea level, leaving it vulnerable to rising seas, storm surges, coastal erosion and saltwater entering freshwater supplies.
Ocean warming, acidification and coral bleaching could further threaten the reefs that help maintain atoll islands. While some studies have found individual islands expanding as waves redistribute sediment, scientists warn that the long term future of atolls will depend heavily on whether coral growth and sediment accumulation can keep pace with rising sea levels.