How are coral reefs structured?

Coral reefs are a complex structure that have been built from the sea floor up.  The lower levels are made of dead coral skeletons.  Wave action and other environmental factors cause the weathering of the reef.  In this manner caves, holes, cracks and crevices are formed.  The hard corals build the structure of the reef and other creatures take up residence, including soft corals, sponges, algae, and seaweed.