The endless waterfall of the Rim. It endlessly flows into space, taking many unfortunate things and creatures with it. The Discworld's water never runs out due to the Disc's magic ("arrangements are made"). Indeed, the fall part is magical to begin with. Our view of the cosmos would suggest that (1) the water would just as soon cling to the other side, and (2) the thin Disc wouldn't generate enough gravity to begin with. Now try to calculate the shape of the gravitational field of four enormous elephants standing on a turtle bigger than the Disc. The magical force involved here seems to beggar any of the local displays described in the chronicles.