Great Barrier Reef, a coral reef in the Pacific Ocean extending about 1,250 miles (2,000 km) along the coast of Queensland, Australia. It reaches from New Guinea in the north to Swain Reefs near the Tropic of Capricorn. The reef is the largest coral formation in the world and probably rests on an old sunken shore line. It consists of detached sections from 10 to 90 miles (16 to 145 km) wide. The reef is broken by many passages that give access to the mainland from the deep water on the ocean side. These passages include Great Northeastern Channel, Flinders Passage, and Trinity Opening.
The reef forms a natural breakwater for a shallow channel, 10 to 100 miles (16 to 160 km) wide, that separates the reef from the mainland. In the channel are many islands, including the Northumberland, Cumberland, and Palm groups, and Whitsunday and Hinchinbrook islands. Mangrove trees grow on the islands, and there are tourist resorts with beautiful coral gardens on some of the larger islands.