Transcript for Inside a Flat Earth convention, where nearly everyone believes Earth isn't round
Here's ab's Eva pilgrim. Let's ask basic questions. The sky is? In a world where nothing is as it seems. The sky is blue. 1 plus 1 is? 2. Reporter: There is at least one truth we thought was indisputable. And the Earth is? Flat. Flat. The Earth is flat. My reality, my senses tell me that the Earth is flat and stationary. Reporter: Or so I thought. For the people attending the first flat Earth international conference here in Carey, north Carolina, their Earth indisputably is not round. Everybody here can agree on absolutely one thing, which is it is not a globe. Reporter: For century, a flat Earth was accepted as certainty until science and sailboats said otherwise. You never admit that there's one bigger than yourself. Reporter: But it wasn't until 2015 when this guy posted his flat Earth clues. Part of a series of clues that help you get around both the design of the flat Earth system we live in. You're kind of like the father of this movement. Don't do that. Reporter: You started it all. I did not invent flat Earth. All I did was walk up to a door, point at it and say you know what, I think there's some really interesting things on the other side of this. And check it out for yourself. If flat Earth is a university, then I would be the freshman recruiter. Reporter: People have traveled around -- actually, mark would say across the flat world to ascend. The first rule of flat club is you don't talk about flat club. Reporter: Until now. I have a poem about that. Reporter: Amy Nicholson wrote a book about her flat Earth journey. I haven't really come out all the way yet. I'm still a little in the closet. This man came from even farther. I mix with quite a lot of flat earthers in New Zealand. You know flat earthers, I guarantee it. But you don't know who they are because they're afraid of talking about it. One, two, three -- We're not crazy! Reporter: Like many modern movements this one has grown in large part out of the internet, with rappers Ivan jellizing to hundreds of thousands of subscribers. No more living on a cartoon ball. Reporter: And YouTube channels like -- Globe busters -- Reporter: Encouraging skepticism about what you've been taught. For the serious students here at the conference -- And this is got to a point now where it's becoming real. When it comes to science, you can test. Fire burns, water is wet, fire burns. Reporter: The curving horizon, the sloping sea level, the spin of the Earth, unless you can see these phenomena with your own eyes, they may not be true. A lot of people are going -- This is crazy, right? But think about this. For the last 20, 25 generations, this is what we told people. Reporter: Unlike what we've been told in school, some flat earthers imagine the Earth looks like a snow globe, round but not a sphere. The north pole is at the center of most flat Earth maps with the ice of Antarctica holding everything in. Are you sure that is what the Earth looks like? Pretty sure. Almost. The basic concept is sound. Absolutely sound. And one thing again, we absolutely know for a fact this ain't it. Reporter: Those iconic blue marble images we see from space, they denounce them as fake. Composite, animation, something that tells you this is not an actual photograph of the Earth. Reporter: So if you think you have questions, the flat earthers have many, many more. Which is why I thought someone who has actually seen the Earth from space might have some answers. 1 plus 1 is? 2. Jimmy: Wh Reporter: What color is the sky? Blue. Reporter: And the Earth is? Round. Reporter: This professor spent more than 20 days in space and worked on the Hubble Telesco telescope. Reporter: When you looked at the planet, what did it look like? It looks round, folks. It is round. My eyewitness account. And I looked at it as much as I could. It is round. We're in the final countdown. How you doing over there? Reporter: He's played himself on "The big bang theory" but some flat earthers actually believe astronauts are just actors, part of a huge conspiracy going back to those very first steps on the moon. That's one smarl step for man, one giant leap for mankind. You don't believe they've gone to space? No. The rockets go up, sure. There's nobody in them. Reporter: Rob skiva is another up with of the celebrity contrarians in the flat Earth movement. His popular YouTube videos and podcasts take clues from the bible's book of genesis. Reporter: There's no way you can get a spinning heliocentric globe in the bible. I have become skeptically of everything. Rightfully so. Pretty much everything here is a conspiracy theorist. That's a common trait. But there's little bit conspicy theorist in all of us. . Reporter: It's why many of them do their own experiments. There's going to be some sciency people who say that is a real bogo experiment. They have. Reporter: That's a little basic. I know. It's so simple it will go right over your head. Reporter: The credo for many of these believers is check it out for yourself. Go out and test. Reporter: You just want them to ask the questions. Why do you believe what you believe? And go from there? I could tell you something and you just look at me and say I'm crazy. Reporter: A lot of people watching this will say that man is crazy. I'm sure they probably do. I would say well, they're crazy for not testing what they think they believe. Small, for something as clear as this horizon, the interpretation could be infinite. For "Nightline" I'm Eva pilgrim in Carey, North Carolina.
This transcript has been automatically generated and may not be 100% accurate.