1 / 12
The Grand Prismatic Spring, the largest in the United States and third largest in the world, and it's colored bacteria and microbial mats in Yellowstone National Park, Wyoming, June 22, 2011. REUTERS/Jim Urquhart

Yellowstone National Park

Signed into law by President Ulysses S. Grant on March 1, 1872, Yellowstone National Park attracts 3.5 million visitors every year and employs over 400 seasonal employees.