I have this story about a medieval general getting besieged in a fortified town. The town is well fortified but the troops defending it, while enough to repel most determined attacks, are not enough to attempt a breakthrough; because of that, after several attempts by the attacking army to take the city, they decided to just starve the defenders out.
The real question
Knowing that his army would probably never be relieved, my general decided to just put the city's populace to work digging a really long tunnel to the outside world. I assume that the tunnel would advance by about 10 meters a day and the city would have enough provisions to last about 1 year; that would mean you get 3.5 km of tunnel, far enough to get out in a nearby wood near the city and evacuate or smuggle in supplies without the attackers noticing anything. It sounds feasible, but I haven't heard anyone doing it in our history, so my question is, why not? Did this ever occur in history? I mean I would do it even preventively, because yes, even if discovered, I can just collapse the tunnel or just easily defend it...
Edit: The town has enough wood to make all the supports and beams needed for the tunnel and space enough to accommodate all the dug-up soil and rocks.