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.
-- from https://en.wikipedia.org/wiki/Crop_circle
While the date printed on DP1 is 31 July 1998, the timeframe for these events is 1992-1993.