Minecraft wiki entry for Shelters, associated text: You never know, a creeper might one day get you. Build a tunnel to escape it. It could also be used to lead to a secondary base. Another idea is to have the tunnel lead to a central room with several other tunnels branching off of it, with tunnels leading to the surface, caves, perhaps even a secret hideout. If playing on a multiplayer server, maybe even add a few dead-end tunnels to confuse anyone who may be following you through the tunnels.