They have branches like satellite dishes that spread out to catch sunlight. Over 80 million square miles of the Earth are covered by coral reefs. That is more than 25 times the size of the United States. The largest coral reef is the Great Barrier Reef off the eastern coast of Australia. It stretches for 1250 miles.
Some coral reefs are thousands of feet thick.
There are four major types of reefs. The fringing reef grows on rocks along a shore. A sandy lagoon separates the barrier reef from land. An atoll is an irregularly shaped ring of coral with a lagoon in the middle. Atolls usually form on top of submerged volcanoes.