Rapid City is the second-largest city in South Dakota and the county seat of Pennington County. Known as the "Gateway to the Black Hills" and the "City of Presidents", it is split by a low mountain ridge that divides the western and eastern parts of the city. Ellsworth Air Force Base is located on the outskirts of the city. Camp Rapid, a part of the South Dakota Army National Guard, is located in the western part of the city. The historic "Old West" town of Deadwood is nearby. In the neighboring Black Hills are the popular tourist attractions of Mount Rushmore, the Crazy Horse Memorial, Custer State Park, and Wind Cave National Park.