Transcript for Controversial YouTube star Logan Paul documents Flat Earth conference
My name is Logan Paul, and I, I think I'm coming out of the flat Earth closet. Reporter: Yes, you heard that right. Controversial YouTube star, Logan Paul laying it down in a trailer for his film about the flat Earth movement. Do you think the Earth is flat? The Earth is flat. I believe the Earth is flat. Reporter: Internet headlines declaring YouTube's baddest bad boy had finally lost it, but it was all a prank. Now that it's all over, do you think the Earth is flat? No, I don't think the Earth is flat. I think that's dumbest I've heard in my life. Reporter: It's a flip satire, following his friends to a flat Earth conference where he tries to convince the world that the Earth is not round. Several hundred attended the conference some thinking he was sympathetic. Some saying it was not funny. I honestly did believe he was genuine. Reporter: Let's get one thing the Earth is round. You know it, I know it. Logan Paul knows it. But his latest stunt highlights the great area between free speech and the power of online platforms to spread misinformation and outright lies. Explain to me what kind of space YouTube is for mistruths. We have to rely on people to say this is ridiculous and seek out information from sources that are verifiable. Reporter: His flat Earth was a joke. But it's a significant truth for a number of believers. In 2017. Eva pilgrim went to the same flat Earth conference Paul went to. And the Earth is? Flat. When it comes to science, there are things you can test, right? Fire's hot, water's wet, those are the things you can test, right? Reporter: But the sloping sea level, the spin of the Earth, flat earthers say unless you can see these phenomenon with your own eyes, they may not be true. A lot of people are going this is crazy. Think about this. For the last 20, 25 generations, this is what we've told people. Reporter: The flat Earth idea seems to have gotten renewed traction in 2015 with this guy. The one rule of power is you never admit there's someone bigger than yourself. Reporter: Mark sergeant posted his alleged flat Earth truth in a series that has had more than 2 million views. Part a series of clues that get your head around the flat Earth system we live in. You're kind of the father of this movement. Don't do that. You're the one who 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 this is a really interesting thing from the other side of this and check it out for yourself. If flat Earth is a university, you know, fea, then I would be the freshman recruiter. Reporter: But, at an actual The Earth is a sphere. Reporter: Jim Davis is a professor at Columbia university's Earth observatory. If you were to ask me why the Earth is round, the simple answer would be gravity. Gravity pulls the same in every direction. It pulls all the matter of the Earth into a sphere. That gravity is the same thing that holds us to the Earth. And so this one sort of simple theory explains just this huge number of things. If you take away one of those pieces, then suddenly, a whole bunch of things don't work. Reporter: Professor Mike massamino spent time in space and worked on the Hubble telescope. When you looked at the planet, what did it look like? It looks round, folks, and I looked at it as much as I could. It is round. Reporter: The mockumentry now has over 5 million views. Logan Paul had a startling rise and has become an internationally-known celebrity. He does a lot of pranks, raucous behavior, your teenage earn likely knows who he is. His most infamous work was blogging a dead body he found in a suicide forest in Japan. Reporter: He said he did it to raise awareness for suicide and suicide prevention. Love him or hate him, his influence is massive. Nearly 19 million subscribers to his YouTube channel. He's raked in $15 million last year from videos, brand deals and merchandise. So when Robbie Davidson, founder of the flat Earth international conference saw Paul's name on the registration list for the 2018 event, he did a double take. I noticed the name and said Logan Paul, I know that name. Reporter: Paul's assistant told him that Paul was curious after a friend told him he was a Davidson says Paul seemed genuine. There were red flags that went up with other people, but I was willing to give him the benefit of the doubt. Reporter: Paul asked to say a few words to the crowd. He still wasn't coming out as a bona fide flat earther. My name's Logan Paul, and I think I'm coming out of the flat Earth closet. When people are really enthusiastic that they have a pretty big-name person in the scene with them. But some other people thought he was there for a hoax. Reporter: Fast forward a few months. The film drops and the parody is laid bare. I felt sick to my stomach. I had the view that it might be neutral. I never, ever thought it would be a positive piece. I thought he humor in it, but I thought he would get people questioning things. It was a mockery from start to finish. Reporter: Did he go too far? Or is it him being him? It is nuanced humor but cruel. It would have been better to examine YouTube's role in pushing the flat Earth theory. Because it's so complicit in and I thought to have a youtuber address his own medium, but that's not the documentary we got. Reporter: Logan Paul did not respond to "Nightline's" request for comment. Do you think it's important to make clear the difference between fact and fiction? Absolutely. We're putting muscle behind in 2019. But unfortunately, it's a little too late. These things have spread so massively that putting that back in the box is all but impossible. Reporter: YouTube says over the past year they have worked to better surface credible news sources across our site tor people searching for news-related topics. And introduce information panels to help give users more sources where they can fact check There are a growing number of people who believe the Earth is flat? It's nothing new. It's in your pocket. While before, the guy knocking on your door clearly had to come to your front door, now it's much easier to access you. Reporter: Yes, we do live in a brave new world, but some truths are as old as time.
This transcript has been automatically generated and may not be 100% accurate.