An atoll is a type of coral reef that forms a ring-like structure encircling a lagoon. These geographical formations are made up of coral debris and often provide a habitat for various marine life. Atolls are typically found in warm ocean waters and are associated with the remains of volcanic islands that have sunk below sea level, allowing the coral to grow upward and create a circular reef.
The word "atoll" comes from the Dhivehi word "atholhu," which refers to a ring-shaped coral reef. It was introduced into the English language in the mid-19th century as explorers and naturalists began to study tropical marine environments.