When did crop circles start appearing?

Answer 1

In one of the earliest records relating to crop circles, British scientist Robert Plot called them "fairy rings" in his book titled The Natural History of Staffordshire, published in 1686. He suggested that they could be caused by airflows from the sky.

Some say that people began making them as a joke, and they are manmade, and duplicated by people using things like a rope and a board. However, although they received considerable publicity since about 1991, they are first recorded in the 1600's :-

In his article "Crop Circles 2010 The Twenty Year Curve" (in "Nexus" magazine Vol. 17 No.6 for Oct-Nov 2010 page 57) Andy Thomas states:-

For something that has been subject to so much dismissal and misrepresentation, the circular puzzle has had an extraordinarly tenacious presence. Although it has its roots going much furthur back, as archive records of circles dating from the 1600's onwards (and photos from the 1930's) clearly demonstrate, it was the early 1990's that saw a surge of new complexity in the fields with "pictograms", after what had just been circles and rings.....

(see links for furthur details)