It is pretty common knowledge that, during the Dark Ages the prevailing belief was that the Earth was flat, and that if you took your ship too far you would fall off the edge into the abyss. This was the prevalent belief because religion repressed any attempt to learn anything, because after all, if you know God created the universe what else matters? Those poor surfs were duped by the oppressive governments and religious powers who championed myth and legend as viable explanations for how the nature of the universe, lest the common man become empowered by knowledge. Thank heavens we live in a time when we don't have to take bullcrap at face value!
L'Image du monde - A 13th Century illustration of the spherical earth.
Except, everything I just wrote about what people in the Dark Ages believed is complete and utter bullcrap. Mankind has known since at least the time of the ancient Greeks that the world is a sphere. What's more, there is absolutely no evidence that western civilization ever lost that knowledge. People have done some awful things in the name of religion, but convincing the uneducated masses that the earth is flat is not one of them.
So where did this silly myth come from? Intellectuals. 19th century, angst-filled intellectuals.
In an effort to discredit creationists many intellectuals such as Washington Irving who asserted in a highly embellished biography of Christopher Columbus, that the Spanish opposed Columbus' "revolutionary" theory of a round earth, on the grounds that such a theory contradicted scripture. Sadly the efforts of people like Irving were incredibly effective, and to this day many people believe that those wacky medieval buffoons actually believed the earth was flat.