-- There are a few undisputed truths that most people agree on: The sky is blue, one plus one equals two and Earth is round. But for a group of flat Earth believers that final point is up for debate.
For the attendees of the Flat Earth International Conference, such as Mark Sargent, "Everybody here can agree on absolutely one thing, which is [Earth] is not a globe," he said.
Sargent, who has a large following on YouTube thanks to his series, "Flat Earth Clues," said he denies that he is the "father" of the movement and rather thinks of himself more like a "recruiter."
"Flat Earth is not something new. I did not invent flat Earth," Sargent said. "All I did was walk up to a door, point at it [and] say, 'You know what I think it's some really interesting things on the other side of this."
The flat Earth movement has grown online, with YouTube channels like Sargent's and others including "Globebusters" as well as "ODD TV," a flat Earth rapper with over 100,000 subscribers.
Last November, Sargent and other flat Earth believers gathered at an Embassy Suites hotel in Raleigh, North Carolina, for the Flat Earth International Conference, an educational seminar where individuals and organizations discuss scientific questions about Earth.
Sargent said he thinks there are more people who believe the theory than just those outspoken on the subject.
"You know flat-Earthers," he told ABC News' Eva Pilgrim. "I guarantee it. But you don't know who they are because they are afraid of talking about it."
Conference attendees Amy Nicholson and Kim Gurley both told ABC News they are more reserved in their beliefs.
Gurley, who traveled from Houston, Texas, to attend the conference, said, "I haven't really come out all the way yet." Nicholson said she wrote a book of poetry about her flat Earth journey, but even her best friend told her she "sounds psychotic."
Laini Inivale, who came all the way from New Zealand to be a part of the experience, said he is able to have conversations about the theory back home. "I mix with quite a lot of flat-Earthers in New Zealand."
Other conference attendees told ABC News they believe there is tangible proof behind the idea.
"When it comes to science, there's things you can test right now," Sargent explained, citing fire, water and gravity. "Think about this, for the last 20, 25 generations, this is what we've told people [about Earth.]"
For centuries, scientists have pointed to evidence of a round Earth in everything from the slope of the horizon to the gravitational pull of the Earth. The claims of a flat Earth are at odds with physics and astronomy.
Some flat-Earthers imagine Earth looks like a snow globe with a dome -- round but not a sphere. According to most flat Earth maps, the North Pole is at the center with the ice of Antarctica holding everything in.
Sargent told ABC News he is "pretty sure" that’s what the flat Earth looks like. "I mean there are some details to be worked out, sure. But the basic concept is sound," he said. "Absolutely sound."
Flat-Earthers denounce traditional and iconic "blue marble" images taken from space as fake, including Rob Skiba.
"As soon as you start looking into the pictures of the globe, you start seeing words like 'composite' or 'animation' or you know something that tells you this is not an actual photograph of the earth," Skiba said. "And at that point you're sort of saying, 'Wait a minute.'"
But Mike Massimino, senior adviser for space programs at the Intrepid Museum in New York City, told ABC News the topic is not up for debate. "Looks round folks. It is round," Massimino said.
The former NASA astronaut added that in his own experience looking at the planet from space, Earth looks round. "[It's] my eyewitness account and I looked at it as much as I could. It is round and it is gorgeous. It's beautiful. It looks like a paradise. I felt like I was I was looking into heaven," he continued.
Many flat-Earthers, including Sargent, believe that astronauts are actually actors who are part of a larger conspiracy.
"No astronaut has ever been and this is going to sound wild when I say this, no astronaut has ever been on a top of a pile of liquid explosives launched off in space," Sargent purported. "Oh, the rockets go up, sure, [but] there's nobody in them."
Skiba believes the conspiracy is rooted in deeper religious implications. "The bigger picture many of us have come to believe is hiding God," Skiba said. "To me, that would be the ultimate motivation," he added.
In his YouTube videos and podcasts, Skiba looks at clues from the Bible in the book of Genesis. "It's describing a snow globe basically. When you break down the text of what it represents, there's no way you can get a spinning heliocentric globe out of anything in the Bible," he said.
Skiba said, "I have become skeptical of everything. And I think rightfully so."
Fellow skeptic and conference organizer Robbie Davidson said he thinks many of the flat-Earthers share a common trait, conspiracy. "This might be a little higher on the spectrum, but I think there's a little bit of conspiracy theorists in all of us. I really do," he said.
Many flat-Earthers believe in testing the theory.
Darryle Marble said he conducted his own in-flight experiment using a leveler to test if the plane was flying parallel to a flat Earth.
"If it were a sphere then the surface of the Earth still would have been curving underneath the airplane while it's flying level," he reasoned. "It’s so simple it'll go right over your head," he said adding that people who have flown planes allegedly told him they "haven’t seen any curvature."
Even celebrities like "Nothing on You" rapper B.o.B. are looking for new forms of exploration for the conspiracy theory. B.o.B. launched a Kickstarter campaign to raise money for research to “try every available experiment and test including but not limited to weather balloons, drones, [and] blimps even."
It's something Sargent and Skiba both think is invaluable to learning more about this theory.
“Go out and test,” Skiba said. “I could tell you something, and then you just look at me like I'm crazy. ... I would say, 'Well, they’re crazy for not testing what they think they believe.'”