An atoll is a ring-shaped island that includes a coral rim that encircles a lagoon. The rim may have coral islands or cays, and atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Atolls form from the remnants of 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.

The word "atoll" comes from the Dhivehi word "atholhu," which means "circular group of coral islets". Charles Darwin coined the term in his monograph, "The Structure and Distribution of Coral Reefs". There are approximately 440 atolls in the world, and they are found in the Pacific, Indian, and Atlantic Oceans.

Atolls are important ecosystems that provide habitats for a diverse range of marine life, including fish, sharks, and sea turtles. They also protect coastlines from storms and erosion and are popular tourist destinations due to their pristine beaches and crystal-clear waters. However, they can also be perilous, as coral reefs surrounding atolls often cannot be seen by boats, leading to countless shipwrecks on atolls over the course of history.