Emerald Pool, Yellowstone National Park, 1935

Yellowstone National Park, established 1872, was America's first national park. Approximately two square miles in area, Upper Geyser Basin contains the largest concentrations of geysers in the park--in fact, nearly one-quarter of all the geysers in the world. Colorful hot springs also exist here, including Emerald Pool, named for its beautiful gem-like color.