This amazing construction is a tropical coral reef. They can be found between 30° north and south of the equator, in places where the water temperature stays nice and warm. They form huge structures over really long periods of time, making them the largest and oldest living systems on Earth!

A reef is actually created from lots of tiny animals – coral polyps – which produce a substance called calcium carbonate that builds up to form an external skeleton. Tropical coral polyps have a small algae growing inside them and it is these that give tropical corals their amazing colours.

But coral reefs don't just exist in hot places – there are cold water corals too, which can survive in temperatures as low as 4°C. Cold-water corals don't have the colour of tropical corals because they do not have the algae living inside them, but they still produce enormous structures.

Coral polyps feed on plankton – small plants and animals that float in the sea current. They catch this food with stinging tentacles – a bit like jelly fish do. During the day, these tentacles are usually tucked away, out of reach of hungry fish. But at night they unfold to catch their prey.

Coral reefs are really important places. Although they account for less than 1% of the marine environment they are home to 25% of all marine life and provide nurseries for about a quarter of the ocean's fish! Coral reefs also provide an important barrier against storms, hurricanes and typhoons.

Humans think that coral reefs will help scientists to create new medicines, a bit like they have with plants from tropical rainforests. Some coral reef organisms are already being used in treatments for diseases like cancer and HIV – amazing!

The problem is that a quarter of the world's coral reefs have been really seriously damaged. Some types of fishing and pollution are two problems that are causing this damage. But climate change is also a big threat as corals cannot survive if the water temperature gets too hot.

WWF has been involved in coral reef conservation since the early 1970s and has carried out over 100 related projects in more than 30 countries. WWF's work involves helping fishermen to change their fishing practices to ways which are less damaging to coral reefs and working with governments to tackle climate change.