Fairy Circles: Mystery attracting scientific study and speculation

Natural phenomena that defy scientific explanation hold a special fascination. This also applies to the mysterious `fairy circles' occurring in Namibia, which have baffled researchers for years.
Theories about the origin of the round, barren patches of earth devoid of vegetation and usually embedded in lush grasslands include the outright absurd (landing sites of alien spacecraft, dancing spirits) and range from the unlikely (landmine explosions, meteorite impacts, cosmic rays) to the plausible like geomagnetic waves, minerals in the soil or disturbances resulting from animals or radiation. After all of these theories were disproved another hypothesis was long considered the most convincing.
According to this proposition the circles are caused by the plant Euphorbia Damarana which contains a poisonous substance known as spurge. Based on that assumption the plant creates the barren patches by releasing toxins into the soil thereby preventing any vegetation from growing even after the plant which was the source of the contamination has died and disappeared.

Further study has however created doubt about the veracity of this proposition. After all, botanists have since extracted soil from beneath Euphorbia plants and proved under lab conditions that a variety of other plants indeed thrive in it. The argument that Euphorbia poisons the earth underneath, thereby preventing vegetation from growing, was thus also refuted.

Since then another theory has emerged which is however also controversial. According to this assumption harvester termites cause the circles which have a circumference of between three and ten meters and occur mostly in the arid central Namib and the western Kaokoveld. Advocates of the termite theory agree that the circles are the result of harvesting activity by these insects which collect grass from a centrally located entrance to their underground burrow and transport it down into their tunnel system. As the grass in the immediate vicinity is depleted the termites begin to harvest grass in ever growing concentric circles thereby increasing the dimension of the `fairy circle'.
This hypothesis is also disputed by botanists on several grounds. Firstly they point to the fact that scientists have only succeeded on a few occasions to locate harvester termites in established `fairy circles'. Secondly they rely on the results of other researchers who were unable to trace any remnants of termite tunnels during excavations performed within some the circles thus making it unlikely that the insects were the cause of the phenomenon.
In addition, sceptics refer to another aspect of the ongoing scientific study that seems to disprove the termite theory: Test have proved that seeds planted in the substratum taken from the circles don't grow even if the soil used is removed from the circle. To many experts this suggests that it is not termites but a peculiar composition of the soil which prevents vegetation from developing in the circles.

Proponents of the termite theory counter this argument with an equally convincing claim. According to their rationale no termites were found in the circles because they had vacated their burrow by the time these were studied. According to this assumption termites only occupy their hideaway for such an amount of time as they can safely harvest grass in the immediate vicinity of the tunnel entrance leading to their underground nest. Once the harvested area around the tunnel entrance has increased to a size where a continued gathering of grass becomes too strenuous due to the distance that needs to be covered to the edge of the circle, the termites leave their burrow.
Regarding the fact that no underground tunnels were ever discovered in any of the studied circles, supporters of the termite explanation point out that these tunnels are not reinforced by the termites. Because of this the passageways slowly disintegrate and fill with sand once the underground burrow is abandoned and are thus no longer visible.

The termite hypothesis also seems to offer an explanation for the fact that plants don't grow in the soil of the circles even if that soil is removed from these barren patches. According to the relevant explanation the soil within the circles is not contaminated by cosmic rays, radioactivity or poisonous plants but simply unfertile.
Using that approach, researchers point out that `fairy circles' only occur in homogenous grasslands with low precipitation and sandy subsoil. Once the grass within a circle has been removed by the harvesting activity of the termites, the cleared area is deprived of the plant material that would normally have decomposed and fertilized the subsoil. The little remaining organic matter is then blown and washed away by wind and rain thus removing the nutrients necessary for plant growth.
Although harvester termites seem like the most plausible explanation for the circles some doubts remain. In the absence of a scientific consensus many people thus continue to prefer the metaphysical interpretation according to which the circles are caused by dancing fairies.