Gravity + magic Also I figure it's sheer pressure that drives the water out of the middle again.
I forgot to do the physics side of it, actually. Should that go in Cosmology, do you think, or should I have a separate "Science" category? I'll cover evolution and the like in it, while I'm at it.
It's definitely inspired by it. In fact, the "warm at the centre, cold at the edge" stuff was in large part worked out to avoid being too close a copy of Discworld.
There could be any sort of planet in this universe. Hell, Discworld itself might be floating out there somewhere...
Thanks! They were both drawn by hand, then fiddled with in Photoshop. Not sure whether you saw it, but I had to adjust the planet diagram to make it look more geological (originally the layers sort of curved inwards instead of following the Bore out, which made no sense).