So, in Tolkein lore, the world was originally flat, with most of the land in the middle (hence Middle Earth). But the Numenorians (men who were rewarded with their own Atlantis-equivalent island for service in the first big war against Melkor, but eventually Power Corrupts etc) tried to invade the uttermost west which was basically Elf Heaven. To put an end to that sort of thing, the creator of the world Bent The World and made it a sphere…but left elves able to treat it like a flat disk. So elves can sail west and reach Elf Heaven, but a man or dwarf or hobbit who sails west will eventually wrap around to the east coast of Middle Earth.