Crop circles have all been proven to be man made and several people have come forward and shown how it's done. No mystery there at all. It's a kind of graffiti.
In fact, several "mysteries" that people have mentioned have been disputed/proven, whatever, some in the last couple of years. You just have to research or like I have stumble on the story.
For example, Loch Ness monster has been disputed. First "photo", the grainy one from years ago has been proven to be a hoax and the person who took it admitted it. They've also recently posted what they think causes the ripples in the water that people have seen. It's an underwater phenomenon, something to do with a fault line. I saw the story a few days ago.
As for JFK, multiple forensic experts have said there was no second shooter and Oswald did it and they have shown how.
I do believe Marilyn Monroe was probably murdered. I think she knew too much about the Kennedy's. They were far from the perfect, clean, upstanding family people want to believe.