Basically I'm hoping (cough splutter) to create an underground secret base using tunnels. I suspect there's no way to join tunnels to extend an (make) underground storage facility? In theory there should be something as subways would have tunnels branching off, but anything I've tried, whilst they will join, will also have a wall blocking the other tunnel. Is there anything suitable? The only other way means not using hills, but form walls and roof from splines, but then that's not underground and hidden. Is there anything suitable I haven't found?