There are many threats to coral reefs. Coral reefs are found only in shallow, warm water. The water temperature is usually constant year round. The reef is made up of thousands of corals. When a coral dies, it leaves its skeleton of calcium carbonate behind.
Young corals attach themselves to the old skeleton and the cycle starts again.
Each new generation is built upon the remains of the previous generation. The actual animal that makes up the physical coral is called a polyp. It is column shaped and attaches itself to the floor of the reef. The top end extends into the water and is free to move about. Thousands of individual polyps form a colony, which in turn forms a reef.