A crop circle is a sizable pattern created by the flattening of a crop such as wheat, barley, rye, maize, or rapeseed. Crop circles are also referred to as crop formations, because they are not always circular in shape. While the exact date crop circles began to appear is unknown, the documented cases have substantially increased from the 1970s to current times. Twenty-six countries ended up reporting approximately ten-thousand crop circles, in the last third of the 20th century, and 90% of those were located in southern England.[1] Many of the formations appearing in that area are positioned near ancient monuments, such as Stonehenge. Nearly half of all circles found in the UK are located within a 15 km radius of Avebury.[2] Formations usually are made overnight, but have also been made during the day. The most widely known method for a person or group to construct a crop formation is to tie one end of a rope to an anchor point, and the other end to a board which is used to crush the plants. More recent methods include the use of a lawn roller.

Some crop formations are paid for by companies who use them as advertising.[3] Other formations are sometimes claimed by individuals or groups without any evidence to support their assertion, usually after undesirable legal repercussions become unlikely.[citation needed]