"The only way anybody can come up with "Flat Earth" is by living in the middle of a continental landmass."

The main cause I could determine was rather a deep trauma of some sort or the other and with the result of them now mistrusting everything mainstream by principle and only now trusting their eyes and "intuition".

And believing the earth is flat is maybe the most anti mainstream position ever.

Edit: and my conclusion sort of was, that the only thing, that would really convince some of them is indeed to let them see it with their own eyes. So maybe I will organize a high altitude baloon trip for some people some day, but personally I also always wanted to get as close to space as possible at least once in my life..