An atoll is a round coral reef that has a lagoon inside it. Atolls develop over a period of millions of years from sunken volcanic islands, when reefs build up around a crater. When these coral reefs emerge from the ocean, they form ring-shaped islands that can be many miles in diameter. Long areas of flat land form the edge of an atoll. The lagoon in the middle is shallow, seldom more than 50 metres deep. Channels between the islands are outlets to the open sea. The outer side of an atoll is bordered by the open sea.
While there are atolls in all the world’s oceans, they mostly occur in the Pacific and Indian Oceans.  The Atlantic Ocean has few atolls, most of them in the Caribbean region. They are only found in the warm subtropical and tropical waters.
Remote atolls in the Pacific Ocean have been used for testing nuclear weapons in the past century.  The United States tested its first hydrogen bomb at Bikini Atoll, which is part of the Marshall Islands. France and other nations conducted nuclear testing near uninhabited Pacific islands.
While some atolls are famous tourist destinations because of their white, sandy beaches, others are remote locations with a native population that lives in poverty. They make a living from fishing and farming.