US Army to fight in “subterranean cities” around the world

Recently the US Army said it was spending over half a billion dollars on training “most of its combat brigades” to fight underground mega-cities, not above them. One army source told Miltary.com there were over 10,000 “large underground military facilities” around the world which were to act as “subterranean cities” and were located under major population centers. The question as to why the US Army needs to train “most” of its brigades to fight underground, seems strange, as well as the fact that there are so many underground mega-cities around the world. I go into this as well as a number of things in this video.