Great Barrier Reef

The Great Barrier Reef is the largest coral reef system on Earth.

It is a massive biological structure located in the Coral Sea. This system sits off the coast of Queensland, Australia. It is separated from the mainland by a deep channel. The reef is so enormous that it can be seen from outer space. It is also the world's largest single structure built by living organisms.

The reef system covers a World Heritage Area of 348,000 km squared. This area contains over 2,900 individual reefs. Additionally, the nearby Torres Strait contains about 750 reefs and shoals across 37,000 km squared.

This massive structure is built by billions of tiny organisms called coral polyps. These polyps work together to create the reef framework. Corals require sunlight to survive and grow. Because of this need for light, they only grow in shallow water. They cannot grow above sea level or in very deep areas. The reef structure grows vertically and expands in diameter over time. Much of the current reef sits on top of limestone platforms. These platforms are relics from previous phases of reef growth.

The history of the reef is tied to changing sea levels. About 24 million years ago, Queensland moved into tropical waters. During this time, erosion from the Great Dividing Range created sediments. These sediments initially made conditions unsuitable for coral growth. About 10 million years ago, sea levels dropped significantly. This change helped the reef substrate build up. Around 20,000 to 6,000 years ago, sea levels rose steadily. As the water rose, corals grew on submerged hills of the coastal plain. By 13,000 years ago, corals began surrounding these continental islands.

The current living reef structure likely began growing about 9,000 years ago. Some researchers estimate the living reef is 6,000 to 8,000 years old.

Different parts of the reef have unique shapes and types. In the north, you can find ribbon reefs and deltaic reefs. These specific structures do not exist in the rest of the system. The middle of the reef is mostly made of crescentic reefs.

These reefs often surround islands like Lizard Island. Fringing reefs are common in the southern sections. These are attached to high islands, such as the Whitsunday Islands. Planar reefs are found near the Cape York Peninsula and Cairns. Most islands on the reef are located on these planar reefs. In 2020, scientists even discovered a new reef 500 meters tall in the north.

Humans have a long connection to this environment. Aboriginal Australian and Torres Strait Islander peoples have used the reef for a long time. It is a vital part of their culture and spirituality. European exploration brought new maps to the region. James Cook was the first European to find the reef in 1770. His ship, the *Endeavour*, ran aground on a shoal near Cooktown. Later, Matthew Flinders mapped the area more accurately in 1802. He gave the system its name. He used terms like "barrier reef" to describe reefs that block waves from reaching the coast.

The reef supports an incredible variety of marine life.

More than 1,500 species of fish live among the corals. You can find clownfish, red bass, and coral trout here. There are also thirty species of cetaceans, which are whales and dolphins. This includes the humpback whale and the dwarf minke whale. Large populations of dugongs also live in these waters.

Seventeen species of sea snake inhabit the warm waters. The reef is a complex ecosystem where many species rely on the coral for survival.

Today, the reef faces serious environmental pressures. Climate change causes warming ocean temperatures. This leads to mass coral bleaching events. During these events, large sections of the reef can die. In 2016, high water temperatures killed huge sections of the northern reef. A 2012 study showed the reef lost more than half its coral cover since 1985. Other threats include pollution runoff and crown-of-thorns starfish outbreaks.

However, there are signs of hope. In 2022, reports showed the greatest recovery in 36 years. This was due to the regrowth of fast-growing Acropora coral.