Among all the scientific questions that have fascinated Man over the centuries, surely there is one that is beyond debate. Planet Earth is round, not flat.
The Ancient Greeks first discovered this fact in at least 200BC, if not 300 years earlier. The Renaissance astronomer Nicolaus Copernicus delivered solid mathematical proof in 1543, reports The Daily Mail.
And we've seen it for ourselves in numerous breath-taking images of Earth relayed from space. So how is it that a growing number of people are starting to reject as baloney the fact that our planet is a sphere?
The Flat-Earthers — a derogatory term used to denote people who are out of touch with the realities of modern life — are back on the march. Thanks to the internet, their numbers have grown to a size unparalleled since the Spanish Inquisition and, inevitably, celebrities are flocking to join. Last month, the former England cricketer, Andrew 'Freddie' Flintoff, declared himself a Flat-Earth enthusiast.
He argues that "evidence suggests the world isn't round", asking: "If you're in a helicopter and you hover, why does the Earth not [rotate under you] if it's round?"
He added: "Why would water stay still if we're hurtling through space? Why is it not wobbling?"
Well, Freddie, these are, indeed, tough questions of the perplexing type that a toddler might ask. But Flat-Earth Flintoff won't be distracted from such quandaries by the offer of a lollipop or soft toy.
He has previously revealed his interest on his Radio 5 show, which he co-hosts with former footballer Robbie Savage, when he said the moon landings could have been faked and that he'd been persuaded by a podcast called The Flat Earthers.
Flintoff plans to attend the next annual meeting of the Flat Earth International Conference in the U.S. next year, having somehow missed this year's event — the first ever, which was completely sold out. Perhaps Flintoff was hovering in his helicopter, waiting to see if North America would come spinning round!
But thousands of Flat-Earthers made it to North Carolina, flying from Britain, New Zealand and Argentina, ignoring the fact that aircraft computer systems are configured to navigate a spherical planet, not a flat one.
Robbie Davidson, the conference organiser, claims the movement is in its best shape since Copernicus annoyingly offered proof that our planet is round. He said: "You're going to see more celebrities and scientists come aboard. This is just the beginning."
Indeed, B and C-listers are queueing up to join, as though Flat-Earth were an astrophysics version of TV's Celebrity Big Brother House.
Shaquille O'Neal, the American former professional basketball player and rapper, declared himself one of the gang in March.
"I drive from Florida to California all the time, and it's flat to me," he declared. "I do not go up and down at a 360-degree angle, and all that stuff about gravity." He and Flintoff should get along well.
In September, another U.S. rapper, Bobby Ray Simmons Jr (better known as B.o.B — he had a number-one hit in 2010 with Bruno Mars), set up a crowd-funding appeal to launch a satellite to send back pictures that would prove the flatness of our planet. Thus far, he has garnered only $6,878 towards his $1million target.
Rap stars, cricketers and Flat-Earth theorists seem an odd mix, but Flat-Earth campaigners have traditionally been a motley bunch since their ideas first came into vogue in the 1870s.
Historians agree that even in the Middle Ages, most authorities accepted that the Earth is round — with only religious hardliners believing that God had created Earth as a flat disc at the centre of the universe.
Victorian Flat-Earth ideas were sparked by a test called the Bedford Level experiment.
In 1838, Samuel Birley Rowbotham, a Biblical Creationist — someone who believes the Earth and the universe are the result of a specific act of divine creation — placed a pair of surveyor's rods six miles apart along a section of the Old Bedford River where it runs through a drainage canal in a straight, uninterrupted line through the Cambridgeshire Fens.
Positioning himself next to one rod, he found he could see the other. This, he said, proved the Earth was flat, because if the planet was round and its surface curved, the other rod should be out of sight.
His 'proof' attracted a small army of converts. In 1870, a supporter called John Hampden issued a public wager of £500 (about NZ$116,000 in today's money) to anyone who could provide incontrovertible proof that the Earth was not flat.
Alfred Russel Wallace, a surveyor and eminent scientist who is credited with (independently of Charles Darwin) developing a theory of evolution based on natural selection, took up the challenge. He knew Rowbotham's finding of flatness was the result of an optical illusion caused by a phenomenon known as "atmospheric refraction".
The illusion happens because the density of air varies throughout the atmosphere and can cause light passing through it to get "bent". This bending can "lift" images near the horizon into view.
By adjusting Rowbotham's original experiment to eliminate atmospheric refraction, Russel Wallace won the bet by showing there was a degree of curvature consistent with the planet being round.
The experiment was refereed by the editor of The Field sports magazine, but Hampden refused to accept the result, claimed Wallace had cheated, and sued him.
A long, messy court battle ensued. Hampden was ultimately jailed, for libel and for threatening murder after he wrote to Wallace's wife: "Madam, if your infernal thief of a husband is brought home some day with every bone in his head smashed to pulp, you will know the reason."
However, the court ruled the bet was invalid, accepting Hampden's claim that he'd retracted the wager. Wallace had to return the £500, but both sides claimed victory. One of Hampden's supporters, Lady Elizabeth Anne Blount, a devout Christian and vegetarian, established the Universal Zetetic Society to prove his point.
The word Zetetic means "inquiring"', though the society's limited horizons were betrayed by the title of its journal: The Earth Not A Globe Review. However, it attracted as members an archbishop, a major-general, various aristocrats and literary names of the day.
The organisation limped into the 20th century, becoming the International Flat Earth Society in 1956. Its organiser, Samuel Shenton, of Dover, was a tireless campaigner and speaker on the subject of a flat Earth, dismissing the first satellite images to show Earth as a sphere in 1960 by claiming: "It's easy to see how a photograph like that could fool the untrained eye."
After Shenton died in 1971, the Society's membership records passed to a Charles Johnson in California, who declared himself "president of the International Flat Earth Research Society of America and Covenant People's Church".
"If Earth were a ball spinning in space, there would be no up or down," he told Newsweek magazine in 1984. Instead, he maintained that the world was a flat disc floating on primordial waters.
He claimed his group had 3,500 members. But when Johnson died in 2001, it seemed the movement would die with him. After all, even the Vatican had admitted the Inquisition (the Catholic Church's institution established to combat heresy) had been wrong in 1633 to force Renaissance scientist Galileo Galilei to renounce his theory — under threat of torture — that the Earth does not move around the Sun, although not until 1992.
Then, along came the internet — and now the worldwide web has become a Flat-Earth forum for cranks around the globe (if you will excuse the rotundist terminology).
In 2004, the Flat Earth Society was resurrected as a web-based discussion forum. It led to the official relaunch of the society in 2009. Its first new member was musician Thomas Dolby (who's 1982 hit single was called She Blinded Me With Science). He doesn't believe the Earth is flat — but he did release an album called The Flat Earth.
A glance at Facebook, that renowned conveyor of questionable truths, shows how strongly the movement has grown.
The Flat Earth Bible Society has 161,000 followers, and the two versions of the Flat Earth Society (they split after a row in 2013) share more than 140,000 members.
In this Donald-Trump era of "fake news", should we be surprised so many people are ready to dispute a fundamental tenet of science?
This rejection is not simply about challenging every perceived authority. It is the ultimate bigoted egotism of the social-media age: people seriously believe that the entire universe revolves around them.
Last year, 52 per cent of British adults told online pollsters Atomik Research the Apollo 11 mission — which took man to the Moon for the first time — was fake.
Apollo astronaut Harrison Schmitt, who set foot on the moon in 1972, responded philosophically: "If people decide they're going to deny the facts of history and the facts of science and technology, there's not much you can do with them. I just feel sorry that we failed in their education."
In a ya-boo-sucks response, the Flat Earth Society said last month: "Astronauts simply don't understand that their word isn't worth anything any more. This isn't the Sixties. We don't fall for your nonsense."
Flat-Earthers believe Nasa faked all pictures of a moon landing and of the Earth viewed from space and that Nasa is lying to conceal the truth that the world is a flat disc in space with a wall of ice around the rim (regularly maintained by Nasa which knows the truth) to stop the oceans pouring over the side.
Flat-Earther Robbie Davidson compares his movement to climate-change sceptics who are mocked for questioning zealots.
With the backing of cricketer Andrew Flintoff and other celebrities, he insists their theory is due for wider serious consideration.
"Pretty soon it'll get to a critical point and scientists will have to come to the table," he said. "It's one thing when it's a bunch of fruitloops online, but now we are getting to the point of seeing major names."
One "major name" who disagrees is Elon Musk, chief of the electric-powered vehicle company Tesla and the private space exploration rocket company SpaceX.
Last month, he teased Flat-Earthers by tweeting: "Why is there no Flat Mars Society?"
The Flat Earth Society replied: "Unlike the Earth, Mars has been observed to be round. We hope you have a fantastic day."
Now Flat-Earthers are determined to try to prove their point by visiting space and confirming our planet is a disc themselves.
"Mad" Mike Hughes, a 61-year-old from California, last month had to abort the launch of his home-made steam-powered rocket, in which he aimed to fly to the edge of space, and prove we inhabit a flat planet.
The details of how he intended to do it are sketchy at best.
Federal officials in the Mojave Desert refused permission for the attempt. Hughes says he has secured another launch site.
But no new date has been announced so far.
Similar plans are being laid by Darrell Fox, from Portland, Oregon, leader of the 976-member Flat Earth v Spherical Earth Facebook group. He aims to send up cameras on high-altitude balloons to prove his point — but after several years' effort he has been unable to gather sufficient funding.
Whether such plans get off the ground is not the point. Flat-Earthers have a new platform and are set on exploiting it.
On the internet, anyone can claim anything, and even this, the world's most gravity-defyingly ignorant conspiracy theory has found a fan base.