The North Koreans built four known invasion tunnels under the DMZ and more are thought to exist. See the story of a US officer killed clearing one of the tunnels — the middle of the page here. North Korea is rife with underground military facilities — from jet runways to artillery emplacements — designed to reduce the impact of US air superiority.
“Eighth Army commander Lt. Gen. Bill Burleson said that Soldiers will need to learn how to operate below the surface to avoid sensors and monitoring devices on the ground level.
‘When you look at the ability to move underground, in some ways the tunnels are completely unseen by other sensors,’ Burleson said during the annual Land Forces Pacific Symposium May 17. ‘We’ve got to be able to do that and fight in terrain. And in cities, some of that involves tactics and techniques. There’s a leader development aspect to that. There’s a [communications] aspect to that.’”