An atoll is a ring-shaped coral reef, island or series of islands. It completely or partially surrounds a body of water called a lagoon. And today I decided to tell you in more detail about these amazing creations of nature. 
1. The word “atoll” comes from the Indo-Aryan - atolhu. This language is spoken in the Maldives, located in the Indian Ocean. The Maldives consists entirely of 26 atolls.
2. If you look at them, it immediately becomes noticeable that they are formed by a more or less round ring of corals, which completely or partially surrounds the lagoon located in the center (the lagoon is not always round). Coral is the skeletal material of a colony of coral polyps. It is created by the oldest multicellular organisms that have lived on our planet for approximately 550 million years.
3. The famous British scientist and naturalist Charles Darwin was the first scientist in the world to present the theory of the formation of atolls. This theory was that the different coral reefs we see are in different phases of formation. In other words, their formation has several phases and also consists of two simultaneous processes: the growth of a coral reef and the gradual subsidence of an oceanic island.
The first phase is the formation of a coral reef around a volcanic island. Over time, due to the growth of corals and the sinking of the volcanic island, it turns into a barrier reef. A certain distance begins to be created between the islands and the reef, which is filled with water.
The last phase is the formation of the atoll. At this time, the island is completely hidden under the surface of the water, and we can only see a ring-shaped coral reef with a lagoon inside.
Thus, according to Darwin, a volcanic island, a coral reef, a barrier reef and an atoll are the same thing, only at a different stage of the biological process.
4. During its formation, the coral does not sink, since it does not stop growing (at the same time, the volcanic island begins to “sink”). The time required for its formation can vary greatly, but according to preliminary calculations by scientists, this process lasts approximately 30 million years.
5. Because corals require tropical temperatures to thrive, atolls are found only in the tropics and subtropics.
6. The largest atoll in the world is Christmas Island (aka Christmas). It is part of the Republic of Kiribati. Its area is 388 km², and the area of the lagoon is 324 km².
7. The most significant difference between an atoll and an island is, of course, the presence of a coral reef, while an island actually consists of a piece of land jutting out into the sea or even a lake or river. Atolls, in general, tend to be older than islands and are almost always round in shape due to their volcanic origin. 
8. Like any other geological formation on Earth, the atoll is destined to disappear. It's only a matter of time. It is enough that the average rate of sea level rise exceeds the average rate of coral growth. That is, corals cannot keep up with rising sea levels.
Unfortunately, this phenomenon is observed in the famous Maldives. The Maldives is one of the lowest countries on Earth. Most of the country (more than 80%) is located at an altitude of no more than 1 meter above sea level. And according to scientists’ calculations, if the level of the World Ocean continues to rise at the same speed with which it is rising now, then the Maldives could disappear under water within 100 years.
9. Their height, on average, does not exceed 3-4 meters above the average level of the World Ocean.
10. They are often hidden by ocean waves. Thousands of ships, from ancient Polynesian canoes to large American warships, have become stranded or wrecked on hidden atolls. The Kon-Tiki trackless log raft, probably the most famous raft in history, fell victim to one of these hidden atolls.
In 1947, under the leadership of the Norwegian archaeologist and traveler Thor Heyerdahl, Kon-Tiki successfully covered 6980 km across the ocean, sailing from Peru to the South Pacific Ocean. The most difficult challenge of the journey was not the waves, currents or trade winds in the open ocean. The biggest problem for Thor Heyerdahl and his five other companions was the atolls. Ultimately, on the 101st day of the voyage, the raft crashed on the second atoll, Raroia. 
11. They are often uninhabited islands. Many of them are remote from populated areas and difficult for humans to access. In the 20th century, this isolation made them attractive as nuclear weapons testing sites for countries such as the US, UK and France. For example, the first hydrogen bomb was tested by the United States of America at Bikini Atoll.
France continued nuclear testing at Mururoa Atoll until 1996, which regularly caused discontent and protests around the world.
12. The most famous island countries consisting of atolls are the Maldives (located in the Indian Ocean) and Kiribati (in the Pacific Ocean). Tourism is a vital part of the economy of these countries. However, most of the atolls inhabited by people are not popular among tourists and are classified among the underdeveloped countries of the world.
If they are not visited by tourists, then it is very difficult for local residents to make money. The thing is that they have practically no natural resources. The soil here is quite poor and in rare cases suitable for farming. Therefore, many goods are imported, and the main occupation of the residents is fishing.
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