If you fly over the south-west corner of Africa, you might see mysterious circles pockmarking the landscape. These barren patches among otherwise grassy terrain are called fairy circles. They almost seem alive, growing and shrinking with a lifespan of 30 to 60 years. Local myths say that they’re the work of supernatural entities, and scientists have tried to come up with better explanations for decades. Perhaps they’re made by grazing ants, or radioactivity, or plants that kill their competitors, or toxic gases released from the ground.

But Norbert Juergens from the University of Hamburg thinks that none of these ideas hold water… unlike the fairy circles themselves.

The circles, according to him, are water traps created by a sand termite. The termites eat all the grass within a circular patch, exposing underlying sand grains that store any falling rainwater. These barren freckles are works of ecological engineering, designed to retain precious water in an otherwise dry land. They’re like desert versions of beaver dams.

Juergens was drawn to the circles through his involvement in Biota Africa, a large project devoted to studying and conserving the continent’s wildlife. Across 40 field trips, he took a careful census of all the creatures living near the circles, across their entire range from Angola through to South Africa. And critically, he checked what was living in the ground below the circles, as well as the land above them.

His found that only one animal lives at every site where the fairy circles exist—the sand termite Psammotermes allocerus. Everything else is restricted to just one small part of the circles’ full range. “It was very obvious that this same termite was the only one that was always there,” says Juergens. “I’m convinced that must be the organism responsible. The only other alternative is that there’s some type of bacteria or fungus.”

View Images Psammotermes alloce rus workers and soldiers. From Juergens, 2013. Science/AAAS

Other scientists have suggested that termites create the fairy circles before, but they pointed the finger at the wrong one—a harvester termite called Hodotermes mossambicus. Zoologists quickly dismissed that idea, since the harvester forages over long stretches of land.

The sand termite is a much likelier suspect. It lives in a network of temporary underground nests, centred beneath a fairy circle. From the surface, they’re invisible. They don’t make the obvious mounds or nests of other species. During the day, they stay submerged to hide from their nemesis—meat-eating ants. At night, they rise to the surface to get rid of their waste, producing soil dumps that are visible around the circles.

Juergens used these patches to measure the density of the termites below. He found that the denser the dumps, the more barren the patch, suggesting that the termites are actively killing grass within the fairy circles by eating their roots. As they nibble away at the margins, they circles widen.

After finding that the soil beneath the circles is extremely humid, Juergens reasoned that the termites garden the plants to conserve water. They stop plants from drawing water up away from the central soils, and also falling rain to rapidly drain into the sand. This explains why the circles only occur in a narrow belt of African desert with very specific environmental conditions: sandy soils and around 10 centimetres of rain each year.

View Images Fairy circles in Namibia’s Hartmann Valley. From Juergens, 2013. Science/AAAS

But Walter Tschinkel, an entomologist from Florida State University who has studied fairy circles, is not convinced, even though he too originally suspected termites. “As far as I know, termites do not eat live grass,” he says. “It is more plausible that the grass died of other causes and was then consumed by termites.”

But P.allocerus isn’t an average termite. It’s a generalist species that does seem to feed on live vegetation, and Juergens has observed them feeding on the roots of plants near the fairy circles. He also rules out the possibility that termites are colonising the circles, not creating them. He has found them in the very youngest circles, where the central patch of grass has only just begun to die. They’re there from the very start.

Tschinkel adds that the termite idea doesn’t explain how termites create the fairy circles’ distinctive patterns, such as their distribution or their changes in size and density across the landscape. But Juergens has an answer for this too. The termites forage in an extended perimeter beyond the circles, as well as within them. Competition between different colonies can account for the circles’ even distribution. Juergens has even shown that the closer two circles are, the smaller they stay, which he interprets as a sign of fiercer competition between neighbouring termite colonies.

He also found that the circles shrink during years of extreme rainfall, since the plants around the borders expand and fill in the barren patches. “That was a great relief for me,” he says. One of the biggest flaws to the termite idea was that P.allocerus lives in eastern parts of Africa that don’t have fairy circles. But Juergens notes that these areas see more rain. Under wetter conditions, the fairy circles naturally fill with grass. “When there’s enough water, you don’t need a fairy circle because they’re water traps,” he says.

View Images Plant roots near a fairy circle with worker termite (2). From Juergens, 2013. Science/AAAS

Juergens sees the water-trap concept as a far better explanation than any of the others. Ants? He can rule ants out, since they’re absent in some areas where fairy circles grow. Mike Picker from the University of Cape Town, who recently championed the ant explanation, agrees with Juergens and will soon publish data to that effect.

Toxic gases? Juergens thinks that the termites might actually be responsible for these. These insects are like little cows, relying on bacteria in their guts to digest the plants they eat. And like cows, they produce methane, carbon monoxide and more. He suspects that the gases that other scientists have detected around the circles are actually termite farts.

And what about the idea that the circles are arise naturally from “self-organising mechanisms”, involving competition between different plants? Mathematical models have shown that such competition can produce bizarre rings of plants. Tschinkel backs this explanation, as do other scientists studying fairy circles like Michael Cramer from the University of Cape Town. But Juergens is particularly scathing about it. “The moment someone can come up with measured evidence and data explaining what these mechanisms are and what’s regulating them, we can discuss it,” he says. “So far, self-regulating mechanisms is just a synonym for fairies.”

“The only way for this question to be properly sorted out is with more thorough investigations,” says Cramer. “The challenge is to be able to artificially create or close up fairy circles.” But that’s easier said than done. “In most cases, you’d just take out the nest and see if things are changing,” says Juergens. But the sand termite nests are so dispersed and temporary that they can be very hard to remove. “That makes it hard to do experiments.”