Atoll

An atoll is a low island found in tropical oceans. The key to understanding it lies in its structure: a ring of coral reef that surrounds a central depression. That depression is not always dry land — it is often a lagoon, which is simply a part of the sea enclosed by the reef.

The central depression can hold different kinds of water. Most commonly it is a lagoon connected to the open sea, but in some cases the reef may fully enclose the depression, trapping water that is fresh, brackish, or highly saline. The exact nature of the water depends on rainfall, evaporation, and whether the lagoon has an opening to the ocean.

• Location: tropical oceans only.
• Composition: coral reefs.
• Shape: low islands encircling a central depression.
• Central depression: may be a sea lagoon, or may enclose fresh, brackish, or highly saline water. …