Ocean Quest: The Race To Save The World's Coral Reefs

Coral reefs are often described as the tropical rainforests of the oceans. But marine biologists sometimes use another analogy: that of the canary in the coalmine. These birds were used by miners as an early warning for lethal gas; corals, too, are extraordinarily sensitive to environmental change. For Nancy Knowlton, a scientist at the Smithsonian Natural History Museum, it's an apt description: "If that's the analogy, then the canary has passed out on the floor of the cage. Coral reefs are potentially immortal. They only have to die if we make them."