"But the Pentagon's new focus on preparing to fight peer militaries such as North Korea, Russia and China changed all that.
An assessment last year estimates that there are about 10,000 large-scale underground military facilities around the world that are intended to serve as subterranean cities, an Army source, who is not cleared to talk to the press, told Military.com.
The Army's Asymmetric Warfare Group -- an outfit often tasked with looking ahead to identify future threats -- told U.S. military leaders that special operations forces will not be able to deal with the subterranean problem alone and that large numbers of conventional forces must be trained and equipped to fight underground, the source said."
There is an estimated 10,000 underground cities throughout the world.