The majority of a coral reef is made of calcium carbonate, which primarily comes from the secretions of tiny marine organisms called **coral polyps**. These polyps build hard skeletons as they grow, contributing to the reef structure. Over time, as polyps die, their exoskeletons remain in place, and new polyps colonize the area, leading to the expansion of the reef. This complex ecosystem is home to numerous marine species and plays a vital role in ocean biodiversity. Coral reefs also provide protection for coastlines and contribute to local fishing economies.