Each year nearly four million people visit Yellowstone National Park -- one of the most popular of all national parks -- but few know the fascinating and complex historical context in which it was established. In late July 1871, the geologist-explorer Ferdinand Hayden led a team of scientists through a narrow canyon into Yellowstone Basin, entering one of the last unmapped places in the country. The survey's discoveries led to the passage of the Yellowstone Act in 1872, which created the first national park in the world.