An atoll is a coral reef that encircles a central lagoon. The types of corals that form atolls live only in warm water, so most atolls are located in tropical regions in the Pacific and Indian Oceans. One or more small islands may form on top of atolls. Although many atolls are uninhabited, some do support human populations. The processes that form atolls take place over millions of years. In modern times, climate change, rising sea levels, and extreme weather all pose risks to atolls.

Salem Press Encyclopedia of Science, 2022. 3p.