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If the ocean suddenly becomes too warm, this can be very harmful to corals. It causes the algae that live inside the coral tissue to be expelled. This process is known as bleaching because it turns the corals white.

If bleaching continues for more than about four weeks, the corals may die.

Storms can damage the reef by causing big waves that break up the coral. This photo shows the damage caused to the Great Barrier Reef by Hurricane Hamish.

Outbreaks of crown of thorns starfish can be very damaging to the reef. These starfish are specialist coral predators and can eat huge areas of the reef.

Nearly half of the coral lost over the past 30 years on the Great Barrier Reef has been because of crown of thorns starfish.

Run off is the chemicals and soil washed out from nearby land onto the reef. You can see large amounts of soil being washed out to sea in this satellite image. This can cause problems from the reef in two main ways.

Soil washed onto the reef can cover the coral and kill it. Chemicals, like fertiliser from farming, or even sewage, can cause algae to grow faster and take over the reef.

Some reefs, such as the Great Barrier Reef, are close to mines. To transport the coal and minerals to other countries, they are often shipped from ports like this one. The shipping lanes and industry can damage the reef as clear channels need to be dug to let the big ships through.

Over 275 million people rely on coral reefs for food and income. Many of these are local fisherman who catch fish on the reef using small nets or fishing lines.

In some places, growing populations or bigger boats that catch fish to sell are harming the reef by catching most of the fish.

There are also examples of destructive fishing, for instance using dynamite or bleach to catch fish.

Tourists can harm the reef if they knock and break the fragile coral. Tourist boats like this one can also harm the reef if they are not careful where they anchor.

Some tourists still take bits of coral home with them as souvenirs.