You might think the flat earth obsession was old hat, knocked into anachronism by a world in which so many people flown around the world in planes, but has more adherents today than ever before.  Kelly Weill's new book Off the Edge looks at the history of the Flat Earth movement and the other conspiracy theories it might have helped to flourish. In a world that rejects facts, every lost argument becomes confirmation for the believers, many of whom pray at the church of YouTube.