Are you fighting a war, and up against a significant barrier? Maybe your opponent has better firepower than you do. Maybe they've erected a ridiculously huge wall. Maybe there are so many landmines that a charge would result in unacceptable levels of losses.
Digging under such fortifications is a possible, and very under-used, tactic. Most military forces don't because it's an exceptional pain in the ass. Way too much work. But when was winning a war ever easy?
The plus side to this is both that they won't see you coming, and, if you've done a good job producing the tunnel, it's very hard to fight it. (Yes, there are bunker-busting explosives, and ways to detect tunnelling, even possibly at a distance. But their supplies are limited, and it's hard to tell where the tunnel even is without some serious heavy duty engineering, done under fire at that.) The minus side is that it's a lot of work, and incredibly slow.
And after the war is over? You could enlarge it into a subway system. Or collapse it and fill in the hole.