N. Korea's Ace in the Hole (11,000 - 14,000 underground facilities)

http://www.latimes.com/news/nationworld/world/la-fg-underground14nov14,1,3714107.story?coll=la-home-leftrail

Like so many worker ants, the North Korean soldiers spent their days underground in a vast labyrinth of tunnels.

Their daily commute involved walking down four steep flights of stairs, then passing through a corridor that went nearly 800 yards into a mountain. They carried tightly sealed cartons, believed to contain raw materials for North Korea's secret weapons programs. Some days, especially if they were being punished, they were assigned simply to dig more tunnels.

K., a North Korean in his 30s, was recruited at age 17 into an elite military unit working for the agency responsible for weapons production. He took an oath to work underground for the rest of his career and was assigned to a cave in remote Musan County in North Hamgyong province, about 15 miles from the Chinese border.

"This is how we hide from our enemies. Everything in North Korea is underground," said K., who described the cave on condition that he be quoted using only his first initial and that certain identifying details be kept vague.