Coral reefs are actually formed by a type of tiny animal known as the polyps. These creatures secrete a hard limestone skeleton which creates the frame for the coral to grow. Corals that branch usually grow very fast, even up to 20-30cm a day while large boulder-like corals only grow a few millimetres per year. The way corals grow, depend largely on the ocean currents and the shape and depth of the ocean floor. Modern reefs are actually a result of newer corals building upon eroded away corals that were left dry after the sea-level drop during the ice age.