Great Barrier Reef
The Great Barrier Reef blankets 137,600 square miles and extends a dramatic 1,242 miles along Australia's northeastern coast, making it the largest group of reefs in the world. The reef began forming more than 30 million years ago and is made up of the skeletons of marine polyps. Four hundred species of living polyps can also be found there, along with 1,500 species of fish, as well as crabs, clams, and other sea life. The area is an Australian national park and is visited by two million tourists a year.