For centuries they've puzzled people by their curious appearance.
They crop up in fields across the globe, in patterns ranging from the simple circle to the DNA double helix.
Right now in America, it's that time of year again.
That's right, it's crop circle season.
This year they're popping up all over the Midwest, with recent sightings in Geneseo, Ill.; Sandyville, Ohio; and Huntingburg, Ind.
And the list goes on.
"But the real question that comes up is, what exactly are they?" said Stan Friedman, a nuclear physicist and author of the book "Crash at Corona: The Definitive Study of the Roswell Incident."
That's the question that always has everyone talking.
From crop circle designers and top-level researchers to the baffled farmers who usually find them, nobody really knows the origin of crop circles.
Remember, as the Gallup Polls tell, three-fourths of people in this country believe in paranormal activity.
Just in case that affects whom you choose to believe.
"There's still a massive amount of people out there who really believe that these things are made by aliens," said John Lundberg, a world-renowned crop circle architect from the United Kingdom.
"The crop circle phenomenon obviously feeds off other cultural mythologies."
Lundberg has been building crop circles for more than a decade in southern England.
Trained in fine art, he says he began building circles as an extension of his artwork.
"When I first went out to start making them, I also sort of thought there might be some extraterrestrial explanation," he said.
Others, including Colin Andrews, a world-leading crop circle expert -- yes, such a thing does exist -- have done extensive research that they say proves that not all crop circles come from humans.
Andrews and his team conducted a study in central-southern England during 1999-2000, which assessed more than 200 circles.
That study showed around 80 percent of crop circles to be man-made with the remaining number unaccounted for.
"We found a lot of evidence that really for a first time separated the nonsense of man-made circles from the real phenomenon," he said.
According to Andrews, crop circles not made by humans exhibit a number of peculiar traits.
He says that the soil from these circles has a higher magnetic reading and that the position of the circle in the field will relate to the color or nutritional value of the individual plants.
Still skeptical? Think aliens have invaded the ABC offices?
"We know that a great number of people are doing these with great sophistication, and they enjoy putting a spanner in the works," Andrews said.
"They do it because they want people like myself to believe that UFOs were leaving landing marks."
Another crop circle expert, Jeff Wilson, the director of the Independent Crop Circle Researchers Association, concurs with Andrews.
"Over the years we've had a bit of experience in evaluating all kinds of crop circles," Wilson said.
"There are telltale signs to help us determine whether they're man-made or not. There are typical hoaxer tools, which leave marks on the ground and so you have to look at how the plants are laid down or broken."
Like his fellow crop circle researchers, Wilson would not say that the circles were made by aliens.
"I don't think there's enough evidence to say what the source cause for crop circles is because it's still a lingering question," he said.
Still, many experts are unprepared to rule out aliens.
"I have no qualms about the possibility that aliens are appearing," Friedman said.
"The fact that people can fake [crop circles] doesn't mean that there aren't real ones. It's in my gray basket."
As for Wilson, he's not so sure.
"Aliens as a source cause for making crop circles are really a media-generated, sensationalized kind of a thing. It's not to say it's not possible. It's just that there's no evidence for it," he said.
"If you're looking for an angle to sell, the mystery is still out there."