Not exactly true. If we lived in a vacuum you would be able to see these great distances. The fact that our atmosphere is full of stuff (oxygen, CO2, nitrogen, lots of water vapor, smog) makes visibility beyond X miles not possible. Sorry I don't know the X, but I've seen it before. And of course it varies based on local conditions for air clarity. That being said - I could believe in undisclosed land masses before I can believe in a flat earth.