The largest living structure, the Great Barrier Reef spans more than 1,200 miles (2,000 km) of islands and submerged reefs. ➡ Subscribe: http://bit.ly/NatGeoSubscribe About National Geographic: National Geographic is the world's premium destination for science, exploration, and adventure. Through their world-class scientists, photographers, journalists, and filmmakers, Nat Geo gets you closer to the stories that matter and past the edge of what's possible. Get More National Geographic: Offi