The Great Barrier Reef is the world’s largest coral reef system. Above the water, green and blue ocean hues can be seen with waves creating white fringes; these signs suggest an underwater world abundant in life. Below the water the experience of light is unique; huge shoals of Giant Parrotfish silhouetted against the surface light, white tips of reef sharks seen in the blue of the deep, and carpets of coral colour brought to life by millions of fish in the shallows. The reef is under significant environmental pressure: the acidification of the ocean is causing coral bleaching. Only a global response to rising CO2 levels will save these sublime reef environments across the planet.