Atoll

An atoll is a kind of island. It is made when a coral reef forms around an island that sinks over many years. In the end, the land is gone, and only the coral reef continues to grow until it becomes an atoll, a group of islands shaped like a doughnut. 
Charles Darwin, who is most famous for his theory of evolution, was the first person to find out how atolls form. He said that volcanoes in the ocean sometimes wear away or sink deeper. Coral growing on a volcano needs to be near the surface, and it keeps growing to stay there.
Most atolls are in the warm parts of the Pacific Ocean or the Indian Ocean.
Some island nations like Kiribati, Tuvalu, and the Federated States of Micronesia have atolls as all or part of their country.