Every time a scientist thinks he’s solved the mystery of the fairy circles, someone else comes up with another answer. The bare, circular patches of land—some as wide as a helicopter landing pad and ringed by a border of tall grasses—freckle the landscape from Angola to South Africa. In 2012, a researcher claimed the circles were “alive,” finding that they appeared and disappeared at regular intervals; he saw no evidence that insects or lack of nutrients were causing the formations. But in March, another researcher blamed termites, citing termite tunnels and specimens that he found within the circles. Now, there’s a new hypothesis. In a paper published this month in PLOS ONE, researchers measured the size and density of fairy circles in Namibia and fed data on soil chemistry, moisture content, climate, and vegetation into a computer model. The model suggested that competition between plants is causing the bizarre formations. In the harsh desert environment, plants compete for resources belowground, and some don’t survive. When weaker grass dies, its absence facilitates growth of neighboring plants. The vegetation gap expands until it reaches a size that limits competition between grasses. The circles do not appear further west in Namibia, where it’s even drier, suggesting that they are dependent on a certain balance of not-too-much but not-too-little rainfall. Because the circles occur only over this narrow range of rainfall, short-term changes in precipitation may cause the circles to appear and disappear. Case closed? Not quite. To definitively prove the culprit, researchers will need to go into the field and tinker with fairy circle variables such as moisture and soil chemistry. For now, the mystery still endures.