Interestingly, over decades, speculations about these crop-circles blamed them on the then unknowns that caught the imaginations of the populace -- animal activity, devil himself, aliens, UFOs and so on. Also, the crop-circle patterns in respective times morphed to reflect objects of then modern scientific fancy -- starting with simple patterns of circles and squares, they moved on to straight lines (tougher to explain as due to natural causes) and even the fractal Koch curve, Julia and Mandelbrot set.

It is one phenomenon where the hoax and science kept upgrading their side to keep the mystery of their origins and cause frustratingly on balance between outright hoaxes and genuinely unexplained.

Here is one such anecdote from the recent article by Richard Taylor in Physics World (free registration required):

As the debate raged, some scientists continued to seek alternative natural explanations. One of the most prominent was Terence Meaden, then a meteorologist and physicist at Dalhousie University in Canada. In 1980 Meaden [...] proposing that the curvature of hillsides in southern England affected the local airflow, allowing whirlwinds to stabilize their positions long enough to define circles in the crop fields.

Such scientific speculations received a severe blow in 1991 when, to the glee of the British media, two unassuming men in their sixties declared that they had been creating crop circles for more than 25 years. Their hobby had begun one summer evening in the mid- 1970s, when artist Douglas Bower recounted a story to his friend David Chorley about an Australian farmer who had reported a UFO rising into the sky and leaving behind a circular “saucer nest”. As Bower and Chorley strolled home from the pub through the English countryside, they created their first imitation nest.

In the process, the pair unintentionally triggered a 15-year duel between art and physics. Bower and Chorley were trying to start a UFO hoax, so when Meaden's meteorological theories of crop-circle formation showed signs of catching on, the pair increased the number of circles in their formations, hoping to demonstrate that they were not weather-related. Meaden, however, proved an inventive (albeit unwitting) opponent. By the time Bower and Chorley went public, Meaden had moved on from mere weather patterns to an electromagneto-hydrodynamic “plasma vortex”, which purported to explain not only the elaborate multi-circle designs, but also the flat farm tractor batteries and eerie lights that coincided with their formation!

Such embarrassing encounters between crop-circle 'art' and 'science', other conspiracy theories and the secretive nature of circle-makers have hampered serious studies of crop circles.

Richard Taylor argues in his Coming soon to a field near you (free registration required), why scientists should nevertheless take these crop circles seriously as finding out how these crop-circle artists create their most complex patterns could have implications for biophysics.

The biophysics connection comes this way: Eltjo Haselhoff, a medical physicist, after a scientific investigation, proposed in his published research that the elongated “pulvini”, the visco-elastic joints that occur along wheat stalks of crop-circles, were a result of superheating from electromagnetic radiation. He found that pulvini on bent stalks within a 9 m-wide circle were elongated compared with undamaged crops in the same field. Although other factors could have caused this swell, they couldn't explain the magnitude of the increase, and the symmetric fall-off from the circle's centre to its edge.

In his 2000 paper (see reference 1 and 2 in the list below) he discusses also about the experimental data published in an earlier paper by Levengood and Talbott (1999) about the pulvinus length in these crop circles.

He conjectures from the circular symmetry of many of the crop formations and several eye-witness reports, mentioning the involvement of 'balls of light' (referred to as 'BOLs' throughout his paper) during the formation of a crop circle (Van den Broeke, personal communication, and Meaden 1991), the nature of the electromagnetic 'point source', rather than a plane wave (as suggested in Levengood and Talbott (1999)).

He goes on to calculate the location of the point source above the ground, to cause the extent of damage in the crop circle. From Fig. 2 of his paper, we gather, the BOL was set at a height of 7.8 m above the centre of the circular imprint, for best fit of the data.

Eltjo Haselhoff reaffirms that the pulvinus length expansion in crop circles reported by Levengood and Talbott (1999) is a thermo-mechanic effect, possibly induced by a kind of electromagnetic point source. Simple hand-made formation of the same crops (i.e. hand-flattened area of crop), he points out, did not reveal the same characteristics.

So, biophysicist interpretation of swollen stalks is that during formation the crops were exposed to microwaves -- from portable magnetrons or microwave ovens. Once superheated with this source, the stalk orientation could be readily sculpted, speeding up circle creation.

Scientists have remained reluctant -- perhaps due to the obvious gullibility and hoaxes that encircle these crop-circles — to take these crop-circles seriously. For them existence of these circles are not outside of scientific explanation and hence doesn't pose a scientific challenge or advancement that require sustained investment of mental faculties and resources.

It seems, if not their existence, the how and why of these circles could offer genuine problems of scientific merit. For instance, scientific investigation into the methods of crop-circle creation could, as Richard Taylor suggests, lead to patented techniques for accelerated crop growth!