There are 58 national parks of the United States operated by the National Park Service, an agency of the Department of the Interior. National parks must be established by an act of the United States Congress. The first of these protected areas, Yellowstone National Park, was signed into law by President Ulysses S. Grant in 1872, followed by Sequoia and Yosemite in 1890. The Organic Act of 1916 created the National Park Service "to conserve the scenery and the natural and historic objects and wildlife therein, and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations." These parks combined protect more than 51.9 million acres (210,000 km2) in twenty-seven states and two territories, and they preserve a variety of resources including canyons, mountains, glaciers, deserts, lakes, caves, forests, and valleys. The newest national park is Great Sand Dunes, established in 2004, which like many other national parks was previously a national monument. (Full list...)