An atoll is a ring-shaped coral reef or series of coral rims that encircle a lagoon either partially or wholly. Atoll and lagoons form a rim around an island that offers protection to the central island. The corals of the atoll sit on top of the rim of an extinct volcano called seamounts which have eroded beneath the waters. Atoll islands are mostly low-lying with an elevation of less than five meters. Atolls are only found in the tropical and subtropical warm waters of the oceans and seas. Most of the world’s great atolls are located in the Pacific Ocean, especially in the islands of Tuamotu, Caroline, Marshall, and the Coral Sea.
Examining 5 of the World's Largest Atolls
Great Chagos Bank
The Great Chagos Bank is the largest atoll in the world, taking up a massive area of 12,642 square kilometers (4,881 square miles). Most of this atoll is located underwater, with the exception of a few small islets which have been designated as strict nature reserves by the World Commission on Protected Areas (WCPA) since 1998. It was first surveyed by Robert Moresby in 1839, who also surveyed the aforementioned North Bank. Great Chagos Bank is currently administered by the British Indian Ocean Territory, an overseas territory of the United Kingdom.
The Reed Bank is the second largest atoll in the world at 8,866 square kilometers (3,423 square miles) in size. It is located in the South China Sea to the northeast of the Spratly Islands. In 2016, the Permanent Court of Arbitration determined that the area was within the exclusive economic zone of the Philippines. However, the economic rights to the area, which is rich in gas and oil deposits, is still disputed between several nations. This is similar to the nearby Spratly Islands, which are contested by Brunei, China, Malaysia, the Philippines, Taiwan, and Vietnam. Exploitation of the aforementioned fossils fuels at Reed Bank has been suspended over the last few years, mainly due to disputes between the Philippines and China.
Macclesfield Bank is the third largest atoll in the world at 6,448 square kilometers (2,489 square miles) in size. It is located in the middle of the South China Sea between China, Vietnam, the Philippines, and Malaysia. The atoll is named after the HMS Macclesfield, a British warship which ran aground around the area in 1804. Currently, the atoll is claimed by both China and Taiwan, although the Chinese are the ones that currently administer it.
North Bank (Ritchie Bank)
North Bank is the fourth largest atoll in the world at 5,800 square kilometers (2,239 square miles) in size. The atoll is located in the Indian Ocean, just to the northeast of the island nation of Madagascar. It, along with Macclesfield Bank and Reed Bank, are atolls that are completely submerged underwater. North Bank, also called Ritchie Bank, sits just north of the Saya de Malha Bank, which is the largest submerged ocean bank on Earth. It was first surveyed back in 1839 by Royal Indian Navy Commander Robert Moresby (1794-1854). The atoll is part of the island nation of Mauritius.
Cay Sal Bank (Placer de los Roques)
Cay Sal Bank, also known as Placer de los Roques, is the fifth largest atoll in the world. It is 5,226.73 square kilometers (2018 square miles) in size and located in the Strait of Florida between the United States and Cuba. Besides being one of the biggest atolls on Earth, it is also the third largest of the submerged carbonate platforms that make up the Bahama Banks. Cay Sal Bank was first visited by Spanish explorers in 1511. In recent decades, the area has been used to monitor drug smugglers and refugees attempting to flee from Cuba.