Also known as "rainforests of the sea", coral reefs offer spectacular sights, as well as supporting wildlife, providing food, jobs and coastal protection for an estimated 500 million people.
But human activities are threatening their survival. Scroll through the gallery to see how our actions are putting coral reefs at risk.EMILY IRVING-SWIFT/AFP/AFP/Getty Images
Rising temperatures: Man-made greenhouse gas emissions are making oceans warmer. In hotter water, corals lose their algae coverings and turn white or "bleached". Bleached coral are not yet dead -- but without their algae they eventually starve.ARC Centre of Excellence
Overfishing: The coral reef's inhabitants are its first line of defense. Fish eat some of the creatures that eat coral, while crabs and shrimp use their pincers to protect their coral homes. Fish also graze on seaweed and algae so that they don't grow to cover the reef.
Indiscriminate overfishing can disrupt these ecosystems, leaving the coral more vulnerable to predators. Fishing boat anchors can also scar and break the corals below.RICHARD A. BROOKS/AFP/AFP/Getty Images
Invasive species: Over the past 25 years, lionfish have colonized Atlantic coastal regions, from their native Indo-Pacific waters. It's thought they were released into the Atlantic by owners who no longer wanted them as aquarium pets.
With fewer natural predators, the species' new Atlantic populations are growing rapidly. They can eat and out-compete native coral reef creatures. Aquarium releases, aquaculture and fishing boats can all unwittingly introduce invasive species to delicate coral environments.KARIM SAHIB/AFP/AFP/Getty Images
Acidification: Seawater absorbs our carbon dioxide emissions, which makes the oceans more acidic.
Corals, like many marine species, protect themselves with skeletons made from calcium carbonate. More acidic seawater makes it harder for corals to build new skeletons, and can even decompose existing skeletons.
MENAHEM KAHANA/AFP/AFP/Getty Images
Pollution: Oil spills, septic waste and pesticides can also disrupt coral reef ecosystems.
Researchers at the University of Queensland, Australia, found that when corals are exposed to chemical pollutants over long periods, their resilience to other stresses - including higher temperatures and ocean acidification - may also decrease.ANGELOS TZORTZINIS/AFP/AFP/Getty Images