Why would anyone want to hide a city in the first place? Well sometimes cities are hidden, not because anyone wanted to hide them, but because they became abandoned over time and then overgrown.
Another reason for a hidden city, is that it was built on top of. Then there are those cities that were actually kept a secret for different reasons.
Usually they were attached to some secret project.
A new ground penetrating radar has found many hidden cities and underground systems. They are not in just one or two places either, they have been located all over the world. Some of the finds were incredible. Tunnels were found under a Mayan pyramid that stretched for over 500 + miles.
This would have allowed the Mayans to escape if invaded and many of them must have. Caverns that had been hollowed out and that are over 15,000 years old and are at least three levels deep, have been found under the Egyptian Pyramids. Who built them and why?