They can be anywhere

Cities rise, humanity lives comfortably in them, the industrial revolution has brought progress, jobs, new services, and with it an increase in the life standards, but the cities are still heavily fortified and the basic instict of humanity still rule, with the nations of the world glaring at each other with hostility and craving each other's natural resources.

Even though the humans are the ruling species of the world, there are still areas were no man has ever set foot in thousands of years, and still there are massive fortresses made of a mysterious black stone that still stand to this day.
What are they doing in there?, who build them?, what is inside of them?, what is their function?