I thought Id post this in the science forum, as whether or not the Earth is flat is sort of a scientific question.
[youtube]https://www.youtube.com/watch?v=_KPMuVi88oA[/youtube]
What’s their response when you tell them they can see that the Earth is fly round by flying the plane around it?
Seemingly otherwise intelligent people believe some of the wackiest things. Don’t ever let one’s “seemingly otherwise intelligent” persona fool you into believing they are sane. They are not!
The world is full of seemingly sane people, but dig down a little and they are insane to the core! 
Then there are the crooked liars that know better, but continue to spew BS for reasons like attention seeking, scamming people out of money, or being accepted as a member of a group, or for pure social reasons like making friends.
Some don’t believe it themselves, but continue to perpetuate the lie for various reasons. For some it’s the same thing as coloring their hair like a rainbow, they just want to be different and seek any attention they can get, good or bad.