Queensland, Australia and the Great Barrier Reef

The Great Barrier Reef, the longest structure in the world created by living organisms, consists of more than 2,500 smaller reefs joined together. The area it covers is approximately as large as the state of Kansas. At least 500,000 years old, the reef can be seen from space and was first mapped by the Apollo 7 astronauts in 1968.

Located at the edge of Australia's continental shelf, the Great Barrier Reef stands in water from 325 to 650 feet (100 to 200 meters) deep. On the landward side is a lagoon. On the seaward side, the continental slope plunges thousands of feet into the deep-ocean basin.

Most of the algae found on the reef live within the bodies of the corals, often leaving the corals to float independently in the water while absorbing sunlight. Red algae, green algae, and kelp grow among the...