Coral bleaching is a phenomenon in which corals lose their distinctive colour because they expel the onecelled organisms which live with them in a symbiotic relationship. If a coral is fully bleached, it will turn entirely white, and die unless the stress on the coral is reduced, allowing the symbiotic organisms to return to the coral. Worldwide, coral bleaching began to increase, which some scientists viewed as alarming in the 1990s. Research shows that it is caused by stress on the coral. A healthy coral reef can sustain a wide assortment of organisms, promoting biodiversity, and if corals die, the creatures which surround the reef may die as well.