The mysterious fairy circles of Namibia crop up unexplained and live for an average of 41 years before disappearing. (Photo: Creative Commons: Stephan Getz)

In the grasslands of Namibia, mysterious patches of soil called 'fairy circles' - round bare spots ringed by taller grasses - form and disappear years later, for no discernible reason. Crop circle enthusiasts may cry conspiracy, but a new study of these strange circles reveals that these oddities have a life cycle that may be related to their surrounding environment.

Biologist Walter Tschinkel of Florida State University has been studying fairy circles since 2005, when he was introduced to them on a trip to Namibia. By comparing satellite photos taken over a 4-year period, he confirmed what some scientists had previously suspected: the fairy circles were alive.

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Several circles appeared and disappeared during that time period, and using the data he had, Tschinkel calculated that most of the smaller circles arise and vanish every 24 years, while larger circles  can last up to 75 years. The overall lifespan of these mysterious spots is 41 years.

"The fact that fairy circles are born, mature and die, brings the dynamic nature of this mysterious ecological phenomenon into focus." Tschinkel said. "Until now, their long life spans made it hard to detect that they are not permanent features."

To confirm his results, Tschinkel crunched data collected from the NamibRand Nature Reserve, reports Wired. The park has sold fairy circles to ecotourists for the past decade for about $50 each. The buyers are essentially adopting the land, similar to star purchasing programs. But because each circle sold is marked with a date of purchase, workers at the reserve were able to photograph the fairy circles and estimate the amount of growth that had occurred since the sale.

From the number of fairy circles that had died or started to die over the past 2 to 9 years, Tschinkel calculated that the fairy circles had an average age of about 6 decades. "It gives me some confidence that we really are talking about a lifespan of about 30 to 60 years," he said.

It's still unclear why the circles occur. When they grow, they reveal the red, sandy soil beneath the grassland and tall grasses form a ring around them a short time later. Tschinkel found that the circles form only on sandy soil with minimal stoniness, and that they don't form on shifting dunes or alluvial fans, where sands are deposited by water. 

He initially thought underground nests of harvester termites may have been the cause, but has found no evidence to support that theory. Other explanations, such as differences in soil nutrients or the death of seedlings by toxic vapors from the ground, have also failed to be proven.

"The why question is very difficult," Tschinkel said, according to The Huffington Post. "There are a number of hypotheses on the table, and the evidence for none of them is convincing."