Coral Reefs, Key Largo, Florida, United States

Healthy tropical coral reefs grow horizontally from 1 to 3 cm (0.39 to 1.2 in) per year, and grow vertically anywhere from 1 to 25 cm (0.39 to 9.8 in) per year; however, they grow only at depths shallower than 150 m (490 ft) because of their need for sunlight, and cannot grow above sea level.
• Materials
As the name implies, the bulk of coral reefs is made up of coral skeletons from mostly intact coral colonies. However, shell fragments and the remains of calcareous algae such as the green-segmented genus Halimeda can add to the reef's ability to withstand damage from storms and other threats. Such mixtures are visible in structures such as Eniwetok Atoll.