Minecraft wiki entry for Mining, associated text: One method for defending a mine is a tunnel collapse system. This essentially has the miner press a button to retract sticky pistons in the walls, pulling torches off the walls. These torches are supporting sand or gravel in the roof, preventing it from falling. When the sticky pistons retract, the torches are no longer attached to the walls, and so they disappear. This allows the sand or gravel to fall, blocking off the tunnel. This can be used in a network style mine to block off the assaulted tunnels while still allowing movement past the blockages. It can also be used to trap assaulting mobs in a "Quarantine Area" and prevent them getting loose in the mine.