I believe the worlds deepest active mine is 2.3 to 3.3 miles deep, considring the earths crust ranges from 20 to 50 miles thick, I would presume based any deeper would not be practical.
As for specifics regarding military bases, I can not be sure. However NORAD, though not deep below sea level is a good example of military infrastructure "underground"
edit on 19-1-2013 by BaneOfQuo because: (no reason given)


