It seems to me that you can't cover all the cases, you just have to figure out how often throughout history they have occured and cover the major fraction of th

It seems to me that you can't cover all the cases, you just have to figure out how often throughout history they have occured and cover the major fraction of them. In any case, here are some of the ideas I liked: Some of the ideas in the original comment thread from 2006 were nice. Make the area inconspicuous, make it extremely expensive to dig up. Why not make it look like a giant concrete dumping ground? To thwart geological surveys, maybe you could figure out a way to stealth the tunnels or arrange the tunnels in the shape of a warning message. Perhaps you should make the area extremely inconspicuous and just trecherous to get to for the technologically unenabled (build rivers, hills etc). That way you ensure that mostly only hikers get there and hopefully don't notice the concealed enterance. Of course, that plan could be thwarted if a group of people seek shelter there because of wartime persecution.