I've seen some of the underground tunnels at the DMZ. Supposedly they have all been detected and intercepted right at the DMZ. Seoul is also a long ways from the DMZ, even if some were undetected, I doubt they go that far. Also, they are barely wide and tall enough to get to small NK soldiers side by side. Might could do covert operations using such, if you dug through undetected, but foolish to think you could get an invasion force snuck in. Some NK military engineer's wild haired scheme if you ask me.
Makes an interesting tourist trap though.
Yes. But looked at in the light of
today's terrorist activities, and how
one Korean looks like the other,
undiscovered tunnels could be key.