Contrary to popular belief, Christopher Columbus did not bravely set off on his voyages in the face of people claiming that he might fall off the end of the Earth. Columbus had enough troubles with navigation without adding that one to the list (you know, the whole unexpectedly hitting America thing). The idea that people in the Medieval period thought that the Earth was flat comes from some good old Renaissance snobbery. Once everyone started thinking that they were terribly smart, they began to invent ways in which everyone from the Medieval period were thick as two short planks. This is largely where we get our erroneous views of "The Dark Ages" from. The myth has further been perpetuated by American culture, depicting Columbus as a revolutionary, flying in the face of medieval ignorance. Since Ancient Greece, people have known that the Earth is spherical. They probably picked up on the fact that people who disappear over the horizon tend to come back again. In fact, the shape of the Earth was so well understood that people were even estimating its circumference in the third century BC. People from the Olden Days would probably laugh at us now, seeing as we're the ones with the Flat Earth Society.