Flat Earthers are the people who still don't believe that the earth's round, despite all the available evidence to the contrary.
In the 21st Century, the term "flat-earther" is used to describe someone who is spectacularly - and seemingly wilfully - ignorant. But there is a group of people who claim they believe the planet really is flat. Are they really out there or is it all an elaborate prank?
NASA is celebrating its 50th birthday with much fanfare and pictures of past glories. But in half a century of extraordinary images of space, one stands out.
On 24 December 1968, the crew of the Apollo 8 mission took a photo now known as Earthrise. To many, this beautiful blue sphere viewed from the moon's orbit is a perfect visual summary of why it is right to strive to go into space. Not to everybody though. There are people who say they think this image is fake - part of a worldwide conspiracy by space agencies, governments and scientists.
Welcome to the world of the flat-earther.
Our attitude towards those who once upon a time believed in the flatness of the earth is apparent in a new Microsoft advert depicting an olden-days ship sailing on rough seas, presumably heading towards the "edge of the world", the advert is part of a $300m campaign aimed at rescuing the reputation of Windows Vista by comparing its critics to flat-earthers.
But are there any genuine flat-earthers left? Surely in our era of space exploration - where satellites take photos of our blue and clearly globular planet from space, and robots send back info about soil and water from Mars - no one can seriously still believe that the Earth is flat?
Flat earth theory is still around. On the internet and in small meeting rooms in Britain and the US, flat earth believers get together to challenge the "conspiracy" that the Earth is round.
"People are definitely prejudiced against flat-earthers," says John Davis, a flat earth theorist based in Tennessee, reacting to the new Microsoft commercial.
"Many use the term 'flat-earther' as a term of abuse, and with connotations that imply blind faith, ignorance or even anti-intellectualism."
Mr Davis, a 25-year-old computer scientist originally from Canada, first became interested in flat earth theory after "coming across some literature from the Flat Earth Society a few years ago".
"I came to realise how much we take at face value," he says. "We humans seem to be pleased with just accepting what we are told, no matter how much it goes against our senses."
Mr Davis now believes "the Earth is flat and horizontally infinite - it stretches horizontally forever".
"And it is at least 9,000 kilometres deep", he adds.
James McIntyre, a British-based moderator of a Flat Earth Society discussion website, has a slightly different take. "The Earth is, more or less, a disc," he states. "Obviously it isn't perfectly flat thanks to geological phenomena like hills and valleys. It is around 24,900 miles in diameter."
Mr McIntyre, who describes himself as having been "raised a globularist in the British state school system", says the reactions of his friends and family to his new beliefs vary from "sheer incredulity to the conviction that it's all just an elaborate joke".
So how many flat-earthers are around today? Neither Mr Davis nor Mr McIntyre can say.
Mr McIntyre estimates "there are thousands", but "without a platform for communication, a head-count is almost impossible", he says. Mr Davis says he is currently creating an "online information repository" to help to bring together local Flat Earth communities into a "global community".
"If you will forgive my use of the term 'global'", he says.
And for the casual observer, it is hard to accept that all of this is not some bizarre 21st Century jape. After all, most schoolchildren know that ships can disappear over the horizon, that satellites orbit the earth and that if you head along the equator you will eventually come back on yourself.
What about all the photos from space that show, beyond a shadow of a doubt, that the Earth is round? "The space agencies of the world are involved in an international conspiracy to dupe the public for vast profit," says Mr McIntyre.
John Davis also says "these photos are fake".
And what about the fact that no one has ever fallen off the edge of our supposedly disc-shaped world?
Mr McIntyre laughs. "This is perhaps one of the most commonly asked questions," he says. "A cursory examination of a flat earth map fairly well explains the reason - the North Pole is central, and Antarctica comprises the entire circumference of the Earth. Circumnavigation is a case of travelling in a very broad circle across the surface of the Earth."
Mr Davis says that being a flat-earther doesn't have an impact on how one lives every day. "As a rule of thumb, we don't have any fears of aircraft or other modes of transportation," he says.
Christine Garwood, author of Flat Earth: The History of an Infamous Idea, is not surprised that flat-earthers simply write off the evidence that our planet is globular.
"Flat earth theory is one of the ultimate conspiracy theories," she says.
"Naturally, flat earth believers think that the moon landings were faked, as were the photographs of earth from space."
Perhaps one of the most surprising things in Garwood's book is her revelation that flat earth theory is a relatively modern phenomenon. Ms Garwood says it is a "historic fallacy" that everyone from ancient times to the Dark Ages believed the earth to be flat and were only disabused of this "mad idea" once Christopher Columbus successfully sailed to America without "falling off the edge of the world".
In fact, people have known since at least the 4th century BC that the earth is round, and the pseudo-scientific conviction that we actually live on a disc didn't emerge until Victorian times.
Theories about the earth being flat really came to the fore in 19th Century England. With the rise and rise of scientific rationalism, which seemed to undermine Biblical authority, some Christian thinkers decided to launch an attack on established science.
Samuel Birley Rowbotham (1816-1884) assumed the pseudonym of "Parallax" and founded a new school of "Zetetic Astronomy". He toured England arguing that the Earth was a stationary disc and the Sun was only 400 miles away.
In the 1870s, Christian polemicist John Hampden wrote numerous works about the Earth being flat, and described Isaac Newton as "in liquor or insane".
And the spirit of these attacks lives on to the present day. The flat-earth myth remains the outlandish king in the realm of the conspiracy theorist. While we all respect a degree of scepticism towards the authorities, says Ms Garwood, the flat-earthers show things can go too far.
"It is always good to question 'how we know what we know', but it is also good to have the ability to accept compelling evidence - such as the photographs of Earth from space."