Zombies. We tend to identify with the zombie apocalypse when you’ve had one of those days. Or weeks. Or years. When the world outside just seems to be, well, a bit of a mess. Chock full of mindless creatures, and no one really knows what to do. Culture, rational thought, all washed away by a sea of zombies.
Libraries are a very good refuge. And a minigun behind the enquiry desk would be nice, too.