It may also be important to think of how the apocalypse came to be, and how long it's been since then. Was it a nuclear war? Was it a cataclysm brought about by a massive scientific disaster? Has it only been days or weeks since the apocalypse, and only now have we gotten the courage to go up and see what the surface looks like now? Has it only been months or years, and some people decided to stay in the Metro system while others went up to the surface to pursue their own interests, either as lawbringers or raiders, or just simple loners, nomads, scavengers, or part of any communities that rose up on the surface after the apocalypse? Has it been decades, if not centuries since the apocalypse, and everyone's been confined to the metro because the surface was still too hazardous, and only now has the air outside become breathable?