A coral reef is essentially a huge, diverse underwater community. Many are built on limestone deposits that have been left behind by living—and decaying — creatures.
An atoll is a very specific type of coral reef. Atolls are circular coral reefs that surround a lagoon completely, or partially. They may also have an island or a handful of small islands in the centre. An atoll is formed around an underwater volcano. When the volcano erupts, it creates piles of lava on the sea floor that will eventually rise above the water level to create the centre island.
Corals will begin to congregate around the volcanic eruption. As thousands and millions of years pass, the volcano that originally created the centre island of the atoll will fall dormant.
However, the coral continues to grow, and attract more and more marine life forms. Most of the world’s atolls are in the Pacific Ocean and Indian Ocean.
