Ya, it seems to flow upward from underground. My point is that it's also a lore-friendly excuse to have crazier weather underground, since Klei doesn't want people to be able to dodge weather effects by going into caves. I like to think of the Don't Starve world as a crazy person's dream, hardened into reality. The core is Maxwell/God/Chair-Sitter's dreaming mind. The dream-goo or nightmare-fuel is something like a mass of willful stem-cell things that create the state of reality. Maxwell's sadism and boredom is also an excuse to shake it up over time, so that the player can't turtle for too long.