Unique features and management:
For a place to be considered for a national park position, there must be some unique features. Typically, this is in the form of endangered/native plants or animals, historical monuments, or just geographical uniqueness that you can't find anywhere else. For example, Mammoth Cave National Park is home to the longest-known cave system in the world (400 miles long). All national parks must be managed to avoid humans messing with the environment, and all national parks, and most nationally protected places, are protected by the National Park Service. The NPS is an agency in the federal government that protects and manages national parks in all 50 states. 424 individual units cover more than 85 million acres of land in the U.S. and U.S. territories.
