Coral grows in the ocean and is made up of millions of miniscule marine creatures known as coral polyps. These polyps build their shell-like, hard, protective homes by secreting calcium carbonate around them. Over time, the polyps die in their homes and become a part of the structure. The next generation of polyps continue the cycle, building their homes atop those of the previous polyp generation, adding to the growth of the ever expanding coral reef. Annually, coral is said to grow between .3 and 2 centimeters per year. It has been noted that a coral reef can take up to 10,000 years to form.