These 8 Hidden Underground Cities Are Groundbreaking (Literally)

What goes up must come down. At least, that’s true as far as cities are concerned. Following the news that super-cities are continuing to expand in size—and predictions that two-thirds of the world’s population will be city-dwellers by 2050—urban centers that are short on space are starting to turn their attention underground.

Source: These 8 Hidden Underground Cities Are Groundbreaking (Literally)