The majority of human beings currently live in urban areas -- and this trend seems set to continue. According to the most recent estimates, two-thirds of humanity will live in urban areas by 2050. However, not all cities are created equally. In fact, countries throughout the world are home to cities that, in one sense or another, do not officially exist. So what exactly are these secret, closed cities. What goes on there? Why are these areas hotbeds of the stuff they don't want you to know?