I met a Flat Earther once. After a bit of back and forth about rotation and surface curvature, he fell back on the inspired argument, “I didn’t used to believe the Earth was flat, but then I did DMT, and it, like, opened my mind, man. I suddenly understood the secrets the Vatican’s been hiding from us. You should try it.”

The shape of the Earth is not a great controversy of our time. Ask a room full of people whether the Earth is round and chances are most of them will say yes – unless your chosen room is hosting a Flat Earth Society meetup. Thanks to the internet’s special power of bringing people with similar interests together, the archaic idea that the Earth is a flat plane rather than a spinning ball has seen a resurgence in recent years. The community that promotes this idea was the subject of the acclaimed 2018 documentary film Behind the Curve, recently released on Netflix.

Behind the Curve, being more about the people who believe in the Flat Earth than the theory itself, makes some small effort to take the idea of the earth being flat as a reasonable supposition by chronicling a couple of Flat Earthers’ experiments aimed at verifying rotation and curvature respectively, but it’s obvious what the film’s stance is. Both experiments are portrayed in the documentary as proving the mainstream scientific consensus – that the earth rotates and is round – but this interpretation is disputed by the experiments’ practitioners and the society itself.

But then, of course they would say that. The documentary’s not-so-subtle subtext is that the Flat Earth community gives its members such a vital sense of belonging and importance that it’s impossible for them to actually change their beliefs. The main subject of the documentary, Mark Sargent, is an average, unremarkable middle-aged man who lives with his mother. Being a spokesman for the Flat Earth movement has given him a level of importance and celebrity orders of magnitude greater than anything he’d previously achieved in his life; he wears a different Flat Earth t-shirt in every scene and drives a car with an ITSFLAT numberplate. That, and his involvement has netted him a “symbiotic” relationship with a woman (Patricia Steere) who would probably otherwise be out of his league. Would he really forgo all that in the face of mere evidence? You can’t just abandon your identity.

A teacher of mine once used “people who believe that the Earth is flat” as an illustration of the difficulty of changing others’ minds. Most people accept that the Earth is round, but it’s a difficult thing to prove, at least in casual conversation. Surface curvature? Can’t see it. Rotation? Flat planes can rotate too. The sun and moon? Just flat surfaces at the top of the sky dome, like in a planetarium. The moon landing? Faked. Astronauts and NASA? Cover-up. Circumnavigation? Possible on a the flat plane, just look at the UN’s logo. Magnetic field? It’s a ring magnet. The day–night cycle? This thing. At least in a casual conversation, there’s an answer for everything. Checkmate, globe shills.

Whether the Earth is flat or round is the sort of thing Sherlock Holmes would labour to forget so that he could preserve brain-space for more practical information. Unless you’re a scientist or an astronaut, the shape of the Earth has no practical effect on your life, and there’s nothing simple you can point to that acts as irrefutable proof either way. The strongest argument against, to my mind, is that humanity is nowhere near organised and unified enough to actually perpetrate a massive cover-up of how the universe works and has no reason to do so. But that sort of thinking hasn’t ever stopped conspiracy theorists. Most Flat Earthers wouldn’t be convinced of the world’s roundness unless you launched them into space, and possibly not even then.