Great Barrier Reef, Australia

Top 2: Great Barrier Reef, Australia

As the only living structure visible from space, the 3,000-kilometer-long coral reef in Australia is the world's largest stretch of coral. It is also a home to numerous dolphins, tropical fish and reptiles. It is one of the seven natural wonders of the world. Unfortunately, the acidity and the global rise in water temperatures have caused the precious coral to vanish, and 95 percent of the 7,000-year-old reef may be dead by 2050, should the warming up process continue.