Crop Circles are what the Muggles mistakenly call the entries in the Contorting Cereals class of the Annual International Wizard Gardening Competition (DP1).
A Ministry of Magic committee reports that Muggles have been concerned by “crop circles” miraculously appearing in fields, not realising that they are part of a wizarding competition. It is suggested that the Contorting Cereals class of the annual competition should be banned by the International Confederation of Wizards (DP1).
A crop circle is a pattern created by flattening a crop, usually a cereal but incidents have been recorded for other vegetation (Wikipedia).
Although the date printed on the first Daily Prophet Newsletter (DP1) is 31 July 1998, the timeframe for this event is 1992-1993.
From the Web
Writing by J K Rowling on Pottermore: The Daily Prophet
Harry Potter Wiki: Crop-related charms