[AgentZero]

Well, obviously, it won't. If the sun actually goes nova. However, at this point that's just the leading theory. If the sun didn't go nova (which it CAN'T!), but did something else that made the surface of the planet uninhabitable, then an underground city at least gives some of the species a slim chance of survival (and not just humanity, mind you, they've stockpiled the DNA of thousands of other species. Coz cloning is just so easy doncha know). And hey, slim's always better than none.