Crop circles are regular geometrical patterns that can occur in crop fields or grasslands. Within the patterns the plants are laid down in specific directions. The plants will often be sharply bent or halfway broken at a specific point near the ground. At this point the stalk can look more or less exploded or scorched as by heat.
The patterns typically consist of circles, rings, bows or other rounded elements laid out in a symmetrical manner and often connected with each other. The whole pattern can be small or large, 5-50 meter in diameter, and sometimes very complicated. Since they first gained broad public attention, they have grown in complexity, and each year seems to have its own fashion regarding geometrical pattern. They are usually produced during the night. They are seen all over the World, but the south of United Kingdom seems to show the highest frequency of this phenomenon.
Many of them have been revealed as hoaxes made by groups of people walking coordinated in the fields with planks under their feet, but not all of them can easily be explained this way.
Whatever causes them, there always seems to have been some force rotating or sweeping in a specific direction and thereby breaking and pressing the plants down. Sometimes people have seen light balls hovering over a place where crop circles have been found afterwards. Sometimes local stormy weather with swirling winds and heavy rain have occurred at the time of formation. Crop circles also tend to form at areas with underground waterways. Here are some possible explanations apart from the obvious hoaxes already mentioned:
Under certain physical conditions, a swirling wind may become stable and then spawn secondary swirls in a regular geometrical pattern, which again can spawn tertiary swirls so that you get a fractal structure of vortexes in the air with interconnections. At the bottom of these vortexes, the plants will be pressed down by the rotating air. Such swirls will also tend to contain electrical gradients that cause discharges, so that they can be seen as lightballs. Such discharges can possibly also cause internal explosions and scorching at specific points in the stalks of the plants.
Temperature gradients or gradients of electric charge from the soil up to some altitude in the air can possibly provide the energy for such swirls. If the energy in the gradient is released smoothly, air will be set in regular circular motion. Small sidewinds from such a vortex will trigger energy to be released in nearby places so that daughter swirls are created.
Another mechanism of formation can occur if the temperature at a spot in the ground is very different from that in the air and surrounding areas. Air will be heated or cooled over that spot. This will cause a wind up from this spot or down towards the spot and air to blow along the ground towards or from that spot. Such winds will always cause the air to swirl at the central point and regularly spaced vortexes in the area around.
Then you have the possibility that the circles are hoaxed, but with sophisticated technology, for example by drones that one lets fly over the fields, or by robotic ground vehicles set down by a drone. The drone or vehicle will move along in a specific preprogrammed geometrical pattern and press the plants down. If the drones used are of the vertical take off and landing type, they will blow air downwards and press the plants down this way. Drones can of course also carry specifically designed tools that force the plants to bow down and lie.
Some private persons or clubs with a solid budget could carry out this activity just for fun or because they get economical gain from visiting spectators or from selling books and films, and farmers may get paid for allowing crop circles to be made in their fields. But you also have the possibility that certain governmental, military or research organizations secretly make crops circles for various purposes.
One purpose of such organizations can be a large scale psychological and cultural experiment. Another purpose can be to create a certain structure in the psychology and culture of people that makes people more easy to lead and manipulate.
A purpose can also be to turn the attention of especially curious individuals away from certain governmental activities over to something that will occupy their curiosity more or less permanently and lead it astray.
Probably the crop circles were originally only a natural phenomenon that local people kept wondering about without a broader public being aware of them. But eventually a broader international public got interested in the phenomenon and speculations that these could be caused by extraterrestrials with important messages for us began to occur.
At this point, individuals, clubs and governmental institutions got the idea that they could secretly make their own crop circles in order to use the public attention for own purpose and gain. By now we probably have a situation where several instances of various kind compete with steadily more sophisticated technology about making steadily more spectacular crop circles, and with embedded messages with steadily stronger psychological effects. Those doing this may operate lonely, but have a mutual gain from the activity of other crop circle makers. And of course the natural phenomenon still occurs.
As already mentioned, many believe that extraterrestrial beings produce the phenomenon in order to signal their presence and leave certain important messages to be interpreted by humans. This explanation may not be impossible. But there is very little within the phenomenon that cannot be explained as natural processes or as the result of human activity with varying degree of technical sophistication, from the quite simple to state of the art drone technology.