Grab....in response to

"But how do you tell them apart from the genuine ones, given that the so-called "experts" can't recognise a human-made one? "

I simply use Occam's Razor...if I know SOME circles are made with ropes and planks by practical jokers, why would I even suspect that the rest are made by alien ships that ALWAYS manage to get away unseen? Those 'experts' may be the same experts who pronounced Piltdown Man to be the missing link...they just don't LOOK hard!(the program I saw stated that there usually WERE some footprints near the crop circles, if one looked carefully, though the basic trick was to enter the field along the compressed rows the farmers use for tractor access, thus making prints rare and hard to see)