atoller

An atoll is a ring-shaped coral reef, island, or series of islets found in tropical and subtropical waters. Atolls typically form around a volcanic island that has subsided or eroded away over time. The coral polyps that build the reef require warm, shallow, clear, and nutrient-rich waters to thrive. As the central island sinks, the coral continues to grow upwards, forming a lagoon in the center surrounded by the reef.

The shape of an atoll is usually a roughly circular or oval ring. The width of the reef can vary significantly, and it may be broken in places by channels that allow water to flow between the open ocean and the central lagoon. The lagoon itself can be shallow or quite deep, depending on the geological history of the underlying landmass.

Atolls are ecologically significant environments, supporting a diverse array of marine life within their lagoons and on the reef itself. They are also important habitats for many bird species. Human populations have historically settled on atolls, relying on the lagoon for fishing and the limited land for agriculture and shelter. However, atolls are particularly vulnerable to rising sea levels due to their low elevation.