(New Scientist) The three main theories to explain the origins of the mysterious “fairy circles” of Namibia have just been dismissed, following an in-depth study by South African researchers. “They still remain a mystery,” says Gretel van Rooyen, a botanist at the University of Pretoria, who headed the team conducting the study. 

Fairy circles are discs of completely bare sandy soil anything from two to 10 metres in diameter. Found exclusively along the western coastal fringes of the Namib desert in southern Africa, they are easy to spot because they are barren in the middle yet have unusually lush perimeters of tall grasses, which stand out from the otherwise sparse vegetation of the desert.

From the time researchers began to take an interest in how they were formed in the early 1970s, three major explanations emerged: termites, radioactive soil and toxic debris left in the soil by Euphorbia damarana, the poisonous milkbush plant.

The radioactive soil theory was easily dismissed after van Rooyen sent samples to the South African Bureau of Standards to be tested for radioactivity and they were all found to be negative. “That would have been the perfect explanation,” she says. “But they found no traces.”

Dead or alive

To check out the poisonous plant idea, van Rooyen’s team located some milkbushes in the desert – living and dead – and took samples of soil from underneath. They tried to grow desert plants on it in the lab and found that the grass species Lolium multiflorum flourished in the soil, showing there was no toxic debris to account for the barren patches.

That left just one proposed explanation – that termites mop up all the seeds on the fairy circles leaving nothing that will grow. “It’s the one most people believe,” says van Rooyen.

But when her team dug for termites, or their nests, they found nothing. “We dug trenches up to two metres deep, but found no signs or remains,” she says.

So where do the circles come from? The researchers have shown that grasses withered on samples of soil taken from the fairy circles themselves, but did better than expected when grown on earth from the lush perimeter, proving that there really does seem to be something different about the two areas of soil.

Van Rooyen is now following up the possibility that toxic elements are somehow deposited in the circles. “But even if we find them, how they came to be there is the next problem.” For the moment, she admits, “we’re left with the fairies.”