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A coral reef is a submerged structure made up of many different corals, or small marine invertebrates. These corals are unable to move and cluster with other corals to form colonies. These colonies then secrete calcium carbonate that causes them to bind together to form the reef. Each of these individual animals is also covered with a hard exoskeleton which makes coral reefs appear very strong and rock-like.
Coral reefs vary in size, type and location around the world but they are very sensitive to water properties like temperature and chemical composition. Nearly all of the world’s coral reefs, particularly the largest reefs, are in the tropics.
The following is a list of the world’s ten largest coral reefs arranged by their length. Note that the last three reefs are listed by their area. The Great Barrier Reef however is the largest reef in the world based on both area (134,363 square miles or 348,000 sq km) and length.
1) Great Barrier Reef

Length: 1,553 miles (2,500 km)
Location: Coral Sea near Australia


2) Red Sea Coral Reef

Length: 1,180 miles (1,900 km)
Location: Red Sea near Israel, Egypt and Djibouti

3) New Caledonia Barrier Reef

Length: 932 miles (1,500 km)
Location: Pacific Ocean near New Caledonia

4) The Mesoamerican Barrier Reef

Length: 585 miles (943 km)
Location: Atlantic Ocean near Mexico, Belize, Guatemala and Honduras

5) Florida Reef

Length: 200 miles (322 km)
Location: Atlantic Ocean and Gulf of Mexico near Florida

6) Andros Coral Reef

Length: 124 miles (200 km)
Location: Bahamas between the islands of Andros and Nassau

7) Zhongsha Islands

Length: 50 miles (81 km)
Location: South China Sea

8) Saya Del Malha

Area: 15,444 square miles (40,000 sq km)
Location: Indian Ocean

9) Great Chagos Bank

Area: 4,633 square miles (12,000 sq km)
Location: The Maldives

10) Reed Bank

Area: 3,423 square miles (8,866 sq km)
Location: South China Sea, claimed by the Philippines