What are reefs?

Appearing as solitary forms in the fossil record more than 400 million years ago, corals are extremely ancient animals that evolved into modern reef-building forms over the last 25 million years ago. Coral reefs are unique (e.g., the largest structures on earth of biological origin) and complex systems. Rivaling old growth forests in longevity of their ecological communities and well-developed reefs reflect thousands and hundreds years of history.

Adapted from : http://www.coris.noaa.gov/about/what_are/