An atoll is a ring-shaped coral reef, island or chain of islands around a body of water called a lagoon. An atoll forms when an underwater volcano spills lava onto the seafloor. Over many eruptions, that volcanic rock piles up. Eventually, it rises above the water. This creates an island. Sea creatures called corals settle around the edges of that island. Their stony skeletons form a coral reef.
Over time, the central island erodes and sinks back down into the sea. Meanwhile, the coral reef remains. Over time, ocean waves break off bits of coral and grind them into sand. That sand builds up, along with other material swept in by wind and waves. This gives rise to a ring-shaped island, or smattering of islands, around the bowl of water, or lagoon, left by the sunken volcano.