A ring-shaped coral island surrounding a lagoon is called an atoll.
Atolls usually develop around a volcanic island in warm, shallow tropical seas. Reef-building corals first form a fringing reef along the island’s coast. As the volcanic island slowly subsides or sea level rises, the reef can grow upward while a lagoon develops between the reef and the sinking island.
This formation sequence was famously explained by Charles Darwin after observations during the voyage of HMS Beagle. Modern geology supports the general subsidence model, while also recognising that sea-level changes, reef growth, erosion, and local geology influence individual atolls.
An atoll is not simply any coral reef shaped like a circle. It normally includes a reef rim, a lagoon, and little or no central volcanic island above water. Many atolls are low-lying and therefore especially vulnerable to storms, erosion, and rising seas.