Another interesting feature of discussion of the antipodes in medieval history: the idea "antipode" literally meant "underfoot" - the idea that there were people on the other side of the earth (over a seemingly untraversable ocean, impassable climes, etc.) was as far-fetched to Aristotle (and from thence, the medievals) as the idea that there might be alien civilizations basking in the sunlight of a star visible from Earth. It was not so much a question of southern or northern - think about sticking a pencil through an apple, or that old joke about "digging your way to China" - that was what struck them as impossible, mostly on the grounds of distance, impassibility, improbability that the decendents of Adam could get that far, and most interestingly -- that if there WERE people over there, they had neither met Jesus (see: distance) nor could they be converted and thus saved, and God would not damn a continent of people to such a fate. (This pragmatic of salvation was also the base of the Church's argument against Giordano Bruno's many-worlds hypothesis).

Of course, the amazing teleporting Jeebus of Joseph Smith solves THAT problem later, but that would have clearly been completely untenable in catholic Church doctrine.

As for what the earth would have toppled into -- this really doesn't make much sense in the medieval cosmos, because the planets (Earth included) were thought to be imbedded in spheres. The Earth was absolutely considered to be immobile and fixed in its place by God all the way up to Copernicus.

You might be interested in Edward Grant's "Planets, Stars, and Orbs: The Medieval Cosmos 1200-1687" (Ed is clearly going for the Long Thirteenth Century, which I'm all in favor of, natch).