The term atoll is derived from the Maldivian word atholu and refers to a coral reef that forms a ring around a central lagoon. Above this coral rim are typically numerous small islets that are composed of coral sand, gravel, and aggregate. These islets, often known by the Polynesian word motu, are low lying and on average may be no more than 3 m (meters) above sea level. There are more than 250 atolls in the world; most occupy the warm tropical waters of the Pacific and Indian oceans, but some are found as far south as the Elizabeth and Middleton reefs in the Tasman Sea. Some atolls are very small, such as Rose Atoll in the Central Pacific, which has a combined land and ...

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