An atoll is a ring-shaped coral island that surrounds a lagoon. Picture a necklace of small islands forming a circle in the middle of the ocean, with calm, shallow water in the center. The islands themselves are made from coral reefs, which are built up over thousands of years by tiny animals called coral polyps.

Atolls form in tropical oceans when coral grows around the edges of a volcanic island. Over millions of years, the volcano slowly sinks into the ocean floor while the coral keeps growing upward toward the sunlight. Eventually, the volcano disappears completely underwater, leaving only the ring of coral islands above the surface. The protected lagoon in the middle can become a calm harbor, perfect for boats and sea life.