As explained on QI, and many other places, almost no-one in human history ever actually thought that the world was flat anyway. The Ancient Greeks knew it wasn't, as did the Babylonians, Romans and mediaeval explorers. When standing on a tall cliff looking out to sea, the curvature of the earth is perfectly visible. The ancients watching their ships disappear over the horizon didn't think they'd fallen off the edge of the earth - especially when those ships returned to port...