[spoiler]There's a lot of questions the movie left unanswered, yeah. It explained a bit about how the gravity of things got inverted in the first place and how the two groups of people started living underground, but then pretty much nothing about how history became so twisted. That's actually my biggest complaint about it.[/spoiler] The thing about falling from a great height and hitting the ground is that there's still a chance of living. But if you fall into the sky, you're just dead. And not even a quick death by blunt trauma as you'd likely die from hitting something, but a painful death by suffocation and decompression.