Just wanted to add my two cents' worth to Lindy's comments:

1) Hoaxers have confessed to creating many crop circles.

2) #1 does not prove that all crop circles are hoaxes.

#2 leads to:

A) All crop circles are hoaxes, and the hoaxers behind the remaining unexplained circles haven't confessed or been fingered.

B) Some of the unexplained crop circles were not created by hoaxers, but by "something else": Aliens or a heretofore unknown mechanism not known to or used by humans such as the hand-waving "energy system" often proposed (a label is not the same as an explanation).

There is clearly a bandwagon effect among crop circle hoaxers.

Bandwagon A) The missing hoaxers were just going along with the gag started by other hoaxers; all crop circles really are hoaxes.

Bandwagon B) Hoaxers were inspired by the "something else" that created "genuine" crop circles; some crop circles are not hoaxes.

A key question that might shed some light on the source of the phenomenon, then, is temporal ordering: Has somebody confessed to or been fingered in the creation of the very first crop circle known to have existed?

If so, then logically it's likely that all crop circles are hoaxes (unless these aliens have a wicked sense of humor). If not, then it's not possible to say one way or another...the question remains open.

Maybe Lindy has some information about the first crop circle?