Many of the more esoteric conspiracy theories are hard to debunk. Lets take the earth if round, prove it. You can bring up many cases, sails disappearing behind the horizon, goemetry, etc. but actually doing these measurements and calculations cold is pretty hard. And whenever you bring up a fact, they will dismiss it out of hand. Tectonic plates, I believe they exist, I have seen the evidence in books. But I doubt I could prove it, I've never really studied geology or anything like that. And if I were to debate someone who believes they weren't real, I would probably lose as they would bring up obscure papers, incidents, etc. which I wouldn't be able to refute well.