Just off Australia’s northeastern coast lies the largest living structure in the world—the Great Barrier Reef. It stretches some 2,000 kilometers (1,200 miles) and was built entirely by coral polyps. These tiny marine organisms live in colonies and build hard skeletons under themselves, forming reefs (see How Corals Build Reefs). Coral reefs provide food and shelter for fish and other animals. Normally, the Great Barrier Reef is bursting with life. But recently, the reef has come under threat—along with others like it around
the globe.
This March, biologists made an alarming discovery: They found that 93 percent of the Great Barrier Reef is suffering from coral bleaching, when the corals that make up a reef turn white. The scientists think many of the corals won’t survive.
“It’s the worst bleaching event in history,” says Ruth Gates, a marine biologist at the University of Hawaii.