Known for its vast size, rich marine ecosystem, and brilliant coloration, the Great Barrier Reef is probably the most famous coral reef in the world. As of today, a vast swath of it has turned a sickly skeletal white. It’s fallen victim to coral bleaching, a phenomenon that results when the ocean waters that reefs call home get too hot—weakening or killing off the coral altogether.

A new survey conducted by Professor Terry Hughes of James Cook University, who used charter planes and helicopters to conduct an aerial survey of the reef, found that as much as 95 percent of the northern section of the Reef—an area spanning 100,000 square kilometers (38,600 square miles)—was “severely” bleached.

Hughes flew over the reef, tweeting out pictures of the bone-white reefs alongside calls to action.