Coral reefs owe their bright colours to tiny algae that live inside the coral and supply it with food. When the surrounding water becomes too warm, the coral expels these algae and turns white, a process known as bleaching. Bleached coral is not yet dead, and it can recover if temperatures fall again within a few weeks. Reefs that suffer repeated bleaching, however, rarely regain their former condition, and the fish that depend on them gradually disappear.