An atoll may be a simple ring of island, or it may be a ring of several islands around a lagoon. In the early 1800s, once it was realized that corals were animals that grew their skeletons only in shallow water, atolls became puzzling features of the tropical oceans. Reef building corals have symbiotic algae that grow in their tissues and help them secrete a calcium carbonate skeleton. They can only grow in the upper 100 meters or so of the ocean where light penetrates. Corals are the main building stuff of coral reefs and are bound together and cemented by algae algae, which also require sunlight to grow.
Atolls are coral islands, made entirely of coral that occur in deep water.
Atolls are a little ring of living coral reef built on a kilometers high pile of dead coral. Charles Darwin figured out how coral atolls form. This idea was perhaps his major contribution from his voyage round the world as in the Beagle in the early 1800s. In some places the earth moves up, in others it moves down. First an island is formed by a volcano, then corals start to grow as a fringing reef around the island.