This reminds me of someone I met who believes the Earth is hollow, and that there is a hole at the North Pole that leads down to an underground world. When he told me that he wants to lead an expedition up there to find it, I suggested that a less expensive way forward would be talking to the hundreds of airplane pilots who fly near the North Pole every day, or downloading publicly available satellite images.
I’m convinced that most of the people in this so-called “movement” really don’t believe what they’re espousing. If you’re in California, for instance, and the sun is starting to set, just call someone on the East Coast and ask where the sun is. When you learn that is completely dark there, you will have proved that the Earth couldn’t possibly be flat.