the world was flat until numenor made war on the undying lands. at that point, numenor sank and the world was made round and the undying lands were placed somehow outside them, so that elves could still sail west along the straight way and get there, but everyone else just sailed west around the globe.

later, tolkien changed his mind about a lot of this and played with it, trying to turn it into an always roundworld (scientifically accurate myth was his goal at this point) but couldnt really figure out how it’d work and he was old and then he died