It's one of the first things you learn in school, from the moment you spin that globe at the front of the classroom. The Earth is a sphere rotating on an axis and revolving around the sun.
But if you ask a self-described "flat-Earther," everything about that statement is false, or at the very least, it's never been proven.
"Most of us would say the same thing, that we laughed at this, we thought this was ridiculous, we believed in the globe," said Robbie Davidson, founder and organizer of the Flat Earth International Conference in Denver this week.
But he said his views changed about three years ago, when he started questioning and investigating for himself.
"When it comes down to it we’re finding out that a lot of what we’re being taught these days is more theoretical science," he said. "You can’t apply it under the scientific method."
Flat-Earth believers reject Eratosthenes and Aristotle, Neil deGrasse Tyson and NASA. They do not believe people ever went to the moon, or that there are any real images taken from space.
"It’s a picture — pictures can be doctored," Davidson said.
He said he came to this conclusion from a Christian creationist viewpoint. He believes in a flat Earth, enclosed by a sort of dome, or firmament, that encompasses everything we can observe in the sky.
He said while he doesn't believe there's a vast government conspiracy trying to hide the truth, he believes mainstream science takes an atheistic view that deliberately hides any creator.
But not all flat-Earth subscribers are religious. Some come to the conferences seeking the science, others enjoy learning about conspiracy theories. Davidson said nowadays, there are a lot of skeptics and more people are questioning what they've been taught.
"When you start seeing pilots and engineers and people of all different walks of life coming out, something is going on," he said.
The Flat Earth International 2018 Conference in Denver is the second such conference Davidson has put on in the U.S. this year. He said Denver has a large flat-Earth community. On sites like Meetup, you can find groups in Fort Collins, Denver and Colorado Springs that hold regular meetings to discuss flat Earth theories.
Davidson said he expects more than 600 people to attend the conference Thursday and Friday. He estimates 80 percent of attendees are already believers. The list of speakers includes the author of the book , "Flat Earth, Flat Wrong," who will engage in a live debate with a flat-Earth believer.
"I just encourage people to come out with a bit of an open mind, laugh for a bit because I laughed at this, but keep an open mind and hear what’s presented," he said. "And I’ll tell you, it’s wild."