They could be caused by any of the following:-
1) There has never been even the slightest hint that aliens even exist, let alone were involved.
2) We know that humans make some of them.
3) We don't know that humans did not make all of them.
4) Occam's Razor says that the simplest answer that works is to be preferred.
5) Since we know there are humans, and don't know there are aliens, the Anthropogenic Crop Circle theory is therefore preferable.
I'll let someone else think up the next five facts...
corn and potatoes
No its an edited image
Dorset
History's Mysteries - 1998 The Crop Circle Controversy 12-4 was released on: USA: 2006
Everyone has always believe that crop circles are made by aliens. But that isn't entirely true. Since, no one has really seen an alien do it, there is no way possible to know that aliens are behind this phenomenon. But, humans are more than capable to make a crop circle. All it takes is a lot of practice a corn field ropes and a board and precise measurements and a crop circle can be made. In the movies it always appears that aliens are behind this event. So there you have it, Crop Circles are not, as so many have claim, attempts at some type of communication by aliens. They are made by humans.
Crop circles are a fact. What is less clear is their origin. Some crop circles are so intricate that it would be virtually impossible for a couple of pranksters with boards and ropes to accomplish them. So the theories keep circulating but so far there is no definate answer.
UFO Files - 2004 Crop Circle Controversy 1-5 was released on: USA: 9 August 2004
It seems they predate written records, but the UK reported crop circles in the late 19th century.
One of the earliest reported crop circle was in Lyon, France in 815AD. In England, a late 16th century woodcut shows a crop circle being made by what was called a 'mowing devil'. As with UFO's, the mystery circles are not a recent occurance.
I tink it is brovin and jonny!
I believe it was in the UK in the late 19th century.
The crop is above th gizzard. It looks kinda' like this: o---o0============ This small circle at the beginning is the front area. The other small circle is the crop, the big one is the gizzard. The = signs represent the digestive tract which goes until the end of the worm.