Grand Prismatic Spring, Yellowstone National Park, Wyoming, USA
Situated on a volcanic plateau that straddles the states of Montana, Idaho, and Wyoming, Yellowstone is the oldest of the national parks in the world. Created in 1872, it covers 3,500 square miles (9,000km2) and contains the world's largest concentration of geothermic sites, with more than 3,000 geysers, smoking cavities, and hot springs. Grand Prismatic Spring, 370 feet (112m) in diameter, is the park's largest hot pool in area and the third largest in the world. The color spectrum for which it is named is caused by the presence of cyanobacteria, which grow faster at the periphery where the temperature is lower. Declared a world heritage site by UNESCO in 1978, and an International Biosphere Reserve in 1976, Yellowstone National Park receives an average of 3 million visitors per year. Five of the world's most frequently visited natural sites are in The United States and Canad, and the continent of North America receives more than 100 million tourists annually - more than 20% of world tourism.