Shan, some of those tunnels were big. I mean big enough to drive a truck through.
Did you ever see Chinese Tunnel through the mountains up to the DMZ near Nightmare Range ? That is impressive too. What I always found amazing was all the railroad and road cuts through the mountains/hills had several huge square concrete blocks up on Y pillars. All ready for demolition to drop them in the cuts. The last 20 miles of South Korea up to the DMZ was pretty much all prepared Battle Positions. Mountains/hills all had trench lines and bunkers. The bunkers had machinegun mounts concreted in place with range cards painted on the concrete. All you had to do was drop yoiur MG in and you were ready to go. The DMZ is amazing. Miles of wire fences with another fence 1o meters behind it. Minefields galore. As new officers there, you learned to navigate your night patrols very accurately. And stay to the time schedule you were assigned.
Good people sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf.
Someday your life will flash in front of your eyes. Make sure it is worth watching.