I've always kind of assumed that their greatest underground tunnel project was to get one of their nukes either as close as possible to Seoul, or to the projected route of advance for South Korean/US armored advance.
Maybe both.
Interesting theory but those tunnels at best are 3 - 4 miles into/under the DMZ, to dig that far without it being noticed by modern day kit (even 90's era) is next to impossible. Seoul is quite a distance from the DMZ (about 45km ish) which is far longer than the channel tunnel which was a HUGE undertaking with specialised digging kit. The NK tunnels that have been found were all probably dug by hand and small scale excavators.
And then there's the scale of the detonation. a buried nuclear blast would vent little energy, expending most of it digging a crater and the tunnel would have to be DEEP to not be detected so its going to do even less. Going off the history of the trio of NK tests lets assume that the device goes off like the first, which may have been a fizzle, so somewhere between 0.7kt to 10kt which by nuclear standards is pathetic, powerful weak stuff. The other one was somewhere between 10 and 20kt of explosive force which = Little Boy territory.
Ground bursting so small a weapon would make a pretty big hole and if the blast broke the surface throw a lot of radiation but damage wise, very little.
You're not gonna get this
http://www.youtube.com/watch?v=UPwSN9gUG5c (5.2 megatonnes of boom)