When I was in high school a teacher told us about the Flat Earth Society. We thought it was a funny joke, even though its founder was serious about it. We thought it was a quaint holdover from more ignorant times, kept alive as an ongoing jab at the scientifically-challenged.
But now, thanks to YouTube, millions of impressionable (and ignorant) young people are being led to believe it once again. Or at least to consider it a premise equal in merit to conventional science.
Type in "Flat Earth" in the YouTube search box. It says - no kidding - "about 4,540,000 results". I haven't looked at them all, but quick perusal of the first few pages shows that this is a popular topic (many have hundreds of thousands of views) that a scary number of rubes seem to take this seriously.
Check out this surprisingly civil conversation between a genuine flat-earther and a reasonable skeptic: