Atolls begin as an underwater volcanoes called seamounts. Over time and many eruptions the seamount grows until it rises above the water and becomes an island. Corals grow around the island and, over geologic time, the volcano goes dormant and sinks, but the coral reefs remain and continue to grow faster than the island sinks, eventually creating an island ring made of coral around a central lagoon where the volcano used to be.

To learn more, check out our online coral reef ecology curriculum: http://www.lof.org/education/portal/