An atoll is a ring-shaped coral reef, island, or series of islets that encloses a lagoon, typically forming after the subsidence of a larger, volcanic island. These formations are generally found in tropical or subtropical regions and represent a specific stage in the geological lifecycle of an oceanic reef system. The formation process involves the growth of coral over time, coupled with the gradual sinking or subsidence of the underlying volcanic structure into the ocean basin. Because the reef continues to grow upward with the rate of subsidence, a ring of coral remains encircling a central, deepening lagoon. Atolls are characterized by their low elevation and reliance on biological construction for their structural integrity, distinguishing them from continental or high-volcanic islands.