The grasslands of Namibia — and to a lesser extent its neighbors Angola and South Africa — are speckled with millions of mysterious bare spots called “fairy circles,” their origins unknown.

Now, a study based on several years of satellite images describes the circles’ life span as they appear, transform over decades, and then eventually disappear.

Writing in the journal PLoS One, Walter R. Tschinkel, the study’s author and a biologist at Florida State University, reports that the circles can last 24 to 75 years.

The circles, which range from about 6 to 30 feet in diameter, begin as bare spots on an otherwise continuous grass carpet; after a few years, taller grass starts to grow around the circle’s perimeter.