Under Their Feet
online.wsj.com
online.wsj.com
If so, http://www.businessweek.com/articles/2012-08-02/the-narco-tu... is a way beter read.
Basically, during the civil war a bunch of miners under the command of General Ambrose Burnside dug a tunnel underneath a confederate fort, and then blew it up.
actually met some of the escapees, in paris, several years ago now. it's not a smart idea to lock up a bunch of mining engineers in an old jail... (iirc they used plastic coca-cola bottles [la leche negra del imperialismo!], end to end, to make a pipe down which they pumped air to breath; they also had rails and a trolley).
http://en.wikipedia.org/wiki/Korean_Demilitarized_Zone#Incur...
The tunnel has large start-up costs but then tiny marginal costs for bulk transport. Beyond a certain expected lifetime, it'd surely dominate aerial costs/risks for cost-effectiveness.
Subs are probably somewhere in-between.