An atoll is a ring (or part of a ring) of coral that forms an island in an ocean or sea.

The largest island of the Kwajalein Atoll.
It is called Kwajalein. This island is closed to the public. The whole island is a US Army missle range. You can see the runway for airplanes takes up most of the 3 mile long half mile wide island. The world's largest airplanes can easily land here. Professor Nibble taught at the school here for 5 years. The major mode of transportation is by bicycle.

Kwajalein is part of the Marshall Island archipelago. It is one of the world's largest coral atolls. It has 97 small islands which surround one of the largest lagoons in the world. The islands of the atoll are completely flat. They are only a few feet above sea level.

The atoll is actually the top of an underwater mountain or dead volcano. The underwater mountain is so close to the surface of the water, that coral animals began to grow on top. Coral needs a lot of sunlight. Being close to the surface gave the coral all the sunlight it needed.

A small island in the Kwajalein atoll. Most of the islands have no people living on them.

Over the years the skeletons of the coral animals built up high enough to break the surface of the ocean. Wind and waves broke up some of the coral to form sand. As floating seeds, like coconuts, washed up on the new island, plants began to grow in the sand.. This process happened at least 97 times on the mountaintop to form the atoll we now call Kwajalein atoll.

Not all of the mountain top was high enough to form an island. Many spots are so close, however, that the coral is dry during low tide. For this reason, a person can walk from one island to another when the tide is low.

The Marshall Island archipelago has several atolls like Kwajalein. They are made up of low lying coral islands. Other archipelagos are made up of high islands. They were pushed up from the earth's crust, and were not formed by coral animals.

Related Terms
island - an atoll is a type of island
archipelago - a group of atolls
volcano - atolls are often formed on the top of volcanoes
lagoon - atoll often form a lagoon
reef - coral reefs often surround an atoll or the atoll may have been formed on top of the reef