<h2>Answer: Atoll</h2>
An atoll is an oceanic coral island, usually in the shape of a circular ring, which may be completely or partially closed. It is also called by this term to the set of several small islands that are part of a coral reef, with an interior lagoon that generally communicates with the sea.
Atolls are formed when a coral reef grows around an oceanic volcanic island, as the island sinks into the ocean and disappears underwater, the growth of the coral increases until reaching the sea level, where the action of the wind and the waves erode it and transform it into sand.
It should be noted that this process can take millions of years.
In this sense, the Maldives, an island country located in the Indian Ocean (southwest of Sri Lanka and a few kilometers from India), has these characteristics.
Its territory is organized in 26 atolls and as a country it is constituted by some 1200 islands.