"While most Muggles refuse to believe that magic exists, they find it difficult to account for certain happenings"
-- Professor Phoebus Penrose's report to the Ministry of Magic (DP1)
Professor Phoebus Penrose headed a committee which produced this report about Muggle suspicions about magic. Among other magical examples, it notes that Muggles have noticed, and had suspicions about, mysterious flying objects, the Loch Ness monster, and crop circles left behind after the Contorting Cereals class of the Annual Wizard Gardening Competition. The report concludes that Muggles are more observant than wizards think (DP1).
Crop Circles have always looked deliberate. Now we know why!
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:
Harry Potter Wiki: Muggles