Hey guys, I hate having to look for help when it comes to plotting, but I'm at a loss here. I have an idea for a novel where mankind has built a massive underground network of tunnels and cities after being forced to leave the surface. I can work with stretching the rules there a little bit. What I need, however, is a reason for them to move down and stay down for a long period of time (several generations) before the time of my novel, when the time has come for them to move back up. Nuclear fallout is out of the question, since no fallout would last 100+ years. And I want something that mankind can eventually recover from, to return to the surface — whether that requires ecological reconstruction or re-terraforming remains to be seen. As overdone as it is, could an impact event (meteor/comet) allow for such a situation? Could an impact event make most of the earth's surface uninhabitable but leave the underground protected? Thanks in advance for any advice.