Crop Circles

Crop circles are geometric designs of crushed or knocked-over crops created in a field. The circles come in different designs varying in difficulty, ranging from simple circles to complex patterns .They have clean-cut edges, and appear to have been made by machines, but the crops are in fact intact and not damaged. Because of this and the fact that they usually appear over night, people are quick to assume that it was the working of extraterrestrials. 

The earliest mention of a crop circle dates back to the 1500s.Mentions of crop circles were sporadic until the 20th century, when circles began appearing in the 1960s and '70s in England and the United States. In the 1980s gained more momentum when three additional crop circles, 60 feet wide, were found in oat crops on a farm in England. UFO researchers were sent out to do investigations. By the 1990s the phenomenon of crop circles has turned into a tourist attraction. It is because of this reason it is very rational to assume that jokester farmers looking to gain media and public attention would make these on their own farms themselves. 

As of now there is no substantial evidence that any crop circles have been a result of extraterrestrial force, but there are many people who still believe in it. They think that the circles are made from the landings of spacecrafts or are messages from aliens trying to communicate with humans. There have been countless claims of people stating they have seen UFO-like lights and have heard strange noises around farms at night.
Other, and more rational theories to the origin of these circles include winds, aircraft, earth energy and humans.

http://science.howstuffworks.com/science-vs-myth/unexplained-phenomena/crop-circle3.htm