I was under the impression that while the Flat Earth Society has many members, many (most? all?) simply join for the laughs. It is difficult to say how many - if any - actually believe in a flat earth.
They used to have a lot more members, but some have fallen off
There used to be a "hollow earth" group too. John Symmes was one of the proponents of the belief there were inhabitable places under the crust of the planet, and that it was peopled. His book about this area (entitled, like the book, "Symmesonia", had some literary influence. It's entrance and exit spots were supposed to be the North and South Pole Edgar Allan Poe set the stage for it's appearance in the South Pole in his only, unfinished novel, "The Narrative of A. Gordon Pym", and it laid the foundation for the plot of Jules Verne's "A Journey to the Center of the Earth." Martin Gilbert has a chapter on this belief and Symmes (as well as the "flat earthers" and their egomaniac leader Rev. Wilbur Voliva) in his book "In the Name of Science" which Dover Publications used to print. It's a fun book to read - he also talks about the "Atlantis and Mu" theories, and a wide variety of other theories (Velakofsky and his "mad" comet theories, "Bridey Murphy", etc.).
There used to be a "hollow earth" group too. John Symmes was one of the proponents of the belief there were inhabitable places under the crust of the planet, and that it was peopled
A similar theory was championed in the latter half of the 20th Century by the eccentric Earl of Clancarty, who published a huge number of Flat/Hollow Earth, UFO, Atlantis and other "New-Agey" books under the name of Brinsley Le Poer Trench. I bought quite a few of them as a kid; completely bonkers, but a fun read for a 10 year-old nonetheless.
Even in the Middle Ages people realised the earth was spherical. There's also the myth that Columbus thought the earth was flat; he didn't, however, he was concerned he might run out of food during his Atlantic voyage , due to a miscalculation of the size of the earth: https://www.washingtonpost.com/blogs...=.a6f4fe3dc6f9