What is the greatest hoax of the twentieth century?

@BlueMetalChick It happened, but the six million people may have been far fewer. I think they can repeat an exaggeration enough times so the public will accept it as fact. I was only thinking about the staged Apollo moon landings I saw criticized on YouTube, but this question has brought up multiple issues. As for human industry causing harm to the environment, I'm very much opposed to it. But I've heard that environmentalists fabricate things to make everything seem much worse than it really is. I had a hard time believing in global warming when I moved to Denver in 2009 and there was a snow storm in October.