19. Area 51, Nevada, USA

While many people have tried to enter Area 51, no one has been successful. As the most well-known prohibited site in America, and the most mysterious, people have made many speculations regarding what secrets it holds. Most people speculate that they are hiding captured aliens or UFOs. 

On official records, Area 51 is a US Air Force and CIA testing territory because of its remote location. If this were true, it makes sense why the place is so secretive, but you can never be too sure that they aren't hiding some creatures from space.