The city of Gladstone near the Great Barrier Reef is the world's fourth largest coal-export hub. Dredges, like one seen here, have turned the harbor brown as they work to expand the coal port. Richard Harris/NPR hide caption

itoggle caption Richard Harris/NPR

Ken Caldeira of the Carnegie Institution for Science takes a water sample during his experiment out on part of the Great Barrier Reef. The water is slightly pink because his team is using a dye to trace an acid-neutralizing chemical as it flows across the reef. Richard Harris/NPR hide caption

itoggle caption Richard Harris/NPR

Heron Island is located on the southern end of the Great Barrier Reef, about 25 miles off the northeast coast of Australia. Ted Mead/Getty Images hide caption

itoggle caption Ted Mead/Getty Images