I'm considering running a quest where the players must deal with a large, powerful, and well organized underground Kobold tribe. They initially find the opening of a tunnel in the middle of a village the kobolds invaded during the night. They dug the tunnel from their home base about a half a mile away so it opened up in the middle of the hamlet so they could attack with minimal detection.
How hard is it to dig a tunnel that is >30 feet underground without having it collapse? Is it even possible if the miners have to dig under a small stream, or under an extremely heavy structure like a thick castle wall? Will the pressure of the earth above the tunnel be diverted to the tunnels' walls, or will special supports be needed for the tunnel to work? In addition, how difficult would it be for the villagers to detect that a tunnel was being dug deep under their homes?