Animation with a gravity flip?

Hi, I vaguely remember this film, probably an animation. It had this
world with two communities at opposite ends of this root thing. When
you climbed down beneath it via the massive root system you would get
to a point where the gravity would reverse. To get to the opposite end
you would have to climb down the first half and then climb up the
second. I know it sounds weird, but anyone that has seen it will know
what I mean. Yes that is all I remember!

What is the title?

(PS typing 'root thing' into imdb didn't help at all.)