Situated on a volcanic plateau that straddles the states of Montana, Idaho, and Wyoming, Yellowstone is the oldest national park in the world. Created in 1872, it covers 9,000 km2 and contains the world’s largest concentration of geothermic sites, with more than 10,000 geysers, fumaroles, and hot springs. Grand Prismatic Spring, 112 m in diameter, is the park’s largest hot pool in area and the third largest in the world. The colour spectrum after which it is named is caused by the presence of cyanobacteria, which grow faster in the hot water at the centre of the basin than at the periphery where the temperature is lower. Declared a Biosphere Reserve in 1976 and a UNESCO World Heritage site in 1978, Yellowstone National Park receives an average of 3 million visitors per year. The continent of North America, which contains the five most visited natural sites in the world, is visited by between 80 and 90 million tourists per year—11 percent of world tourism in numbers, but almost 18 percent in revenue.