Back in ancient history, people did not believe that the world was round. They believed that the Earth was flat. They felt that sailors could literally fall off the Earth if they sailed far enough in one direction. However, part of this idea was due to storms and other factors causing sailors to be lost at sea. Therefore, when these captains and their crew never returned, people believed that they fell off the face of the Earth.
However, today, some people still believe that the Earth is flat. Possibly it is due to the fact that they have to believe what others are saying. There is no direct proof unless you have traveled around the world or you are one of a few people who have traveled into space and have viewed the Earth as round.