Corals get their color from algae that live inside them, but when stressed, often by a big change in water temperature, they expel the algae and turn white. This process is called "coral bleaching," and it can be deadly to coral and extremely widespread. Bleaching killed off half of the Caribbean coral in the U.S. in 2005.
Scientists with Australia's University of Queensland are involved in a long-term, worldwide survey of coral bleaching, called the XL Catlin Seaview Survey. It is meant to establish a baseline of images of coral so scientists can compare coral reefs before and after bleaching hits.