The World's Largest Coral Reefs

Great barrier reef
Daniel Osterkamp / Getty Images

coral reef is a submerged structure made up of many different polyps, or small marine invertebrates. These polyps are unable to move and cluster with other corals to form colonies, secreting calcium carbonate which binds them together to form a reef. They have a mutually beneficial arrangement with algae, which live protected in the polyps and make much of their food. Each of these individual animals is also covered with a hard exoskeleton, which makes coral reefs appear very strong and rock-like. Covering only about 1 percent of the ocean floor, reefs are home to about 25 percent of the world's marine species.

Coral reefs vary greatly in size and type, and they are very sensitive to water properties such as temperature and chemical composition. Bleaching, or the whitening of a coral reef, occurs when the colorful algae leave their coral homes due to temperature or acidity increases. 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 nine largest coral reefs ordered 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.

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Great Barrier Reef

Length: 1,553 miles (2,500 km)

Location: The Coral Sea near Australia

The Great Barrier Reef is part of a protected national park in Australia large enough to be seen from space.

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Red Sea Coral Reef

Length: 1,180 miles (1,900 km) 

Location: The Red Sea near Israel, Egypt, and Djibouti

The corals in the Red Sea, especially in the northern part in the Gulf of Eilat, or Aqaba, are under study because they so far have been able to withstand high temperatures.

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New Caledonia Barrier Reef

Length: 932 miles (1,500 km)

Location: The Pacific Ocean near New Caledonia

The diversity and beauty of the New Caledonia Barrier Reef put it on the UNESCO World Heritage Sites list. It’s even more diverse in species count (it harbors some threatened species) than the Great Barrier Reef.

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The Mesoamerican Barrier Reef

Length: 585 miles (943 km)

Location: The Atlantic Ocean near Mexico, Belize, Guatemala and Honduras

The largest reef in the Western Hemisphere, the Mesoamerican Barrier Reef is also called the Great Mayan Reef and is a UNESCO site containing the Belize Barrier Reef. It contains 500 species of fish, including whale sharks, and 350 species of mollusk.

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Florida Reef

Length: 360 miles (km)

Location: The Atlantic Ocean and Gulf of Mexico near Florida

The United States’ only coral reef, the Florida reef is worth $8.5 billion to the state’s economy and is disintegrating faster than scientists had estimated due to ocean acidification. It extends into the Gulf of Mexico, outside the boundaries of its home in the Florida Keys National Marine Sanctuary.

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Andros Island Barrier Reef

Length: 124 miles (200 km)

Location: The Bahamas between the islands of Andros and Nassau

The Andros Barrier Reef is home to 164 species and is famous for its deep-water sponges and a large population of red snapper. It sits along a deep trench called the Tongue of the Ocean.

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Saya De Malha Banks

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

Location: Indian Ocean

The Saya De Malha Banks are part of the Mascarene Plateau, and the area is home to the largest continuous beds of seagrass in the world. The seagrass stretches across 80–90 percent of the area and coral covers 10–20 percent.

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Great Chagos Bank

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

Location: The Maldives

In 2010 the Chagos Archipelago was officially named a protected marine area, meaning it can’t be fished commercially. The Indian Ocean reef area has not been studied extensively, leading to the discovery in 2010 of a previously unknown mangrove forest.

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Reed Bank

Area: 3,423 square miles (8,866 sq km)

Location: The South China Sea, claimed by the Philippines but disputed by China

In the mid-2010s, China started constructing islands on top of reefs in the South China Sea in the Reed Bank region to increase its foothold in the Spratley Islands. Oil and natural gas deposits are there, as well as Chinese military outposts.