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ROBERT SIEGEL, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

AUDIE CORNISH, HOST:

And I'm Audie Cornish.

A mystery in West Africa has puzzled scientists for years. Strange circles of bare soil appear in grassland. They're commonly called fairy circles. They last for decades until grass finally invades and covers them over. Well, we may finally have an explanation.

NPR's Christopher Joyce reports on how insects are bioengineering thousands of miles of desert.

CHRISTOPHER JOYCE, BYLINE: Fly from Angola down to South Africa and you'll see thousands of these fairy circles down below. They look like moon craters anywhere from two to 25 feet across. Now, it's a pretty sure thing they're not made by fairies. So by what then? Poisonous plants? Seeping gas? No one knew. Then came one very determined biologist: Norbert Juergens. He traveled from the University of Hamburg in Germany to dig trenches inside the circles. In every circle, it was wet underground, even in dry seasons. Juergens, who's not given to excitability, knew he was onto something extraordinary.

NORBERT JUERGENS: It was an exciting observation because water is the most important resource in the desert. So I sort of discovered one million little circular oases in the desert at that moment.

JOYCE: And there was something else odd about the circles. They were ringed by tufts of year-round grass.

JUERGENS: It's a plantation. It's a plantation of plants created by some organism.

JOYCE: But what kind of organism? He found spiders, beetles, ants, even aardvarks in the circles and termites, and only termites showed up underneath every circle. Juergens figured out what these termites were doing. After it rains, new grass grows. Normally, that grass sucks all the water out of the ground and then dies. But if the termites kill the grass first by eating the roots around their nests, the water stays in the ground for years. The termites literally swim in watery sand.

And that outer ring of year-round plants, they stick their roots just inside the circle to get the water but not far enough to tempt the termites. The reason no one figured this is out before is that the termites mostly move about at night and don't build big noticeable nests. Juergens says as ecological engineers, these termites are mighty. They create oases where lots of animals can live. They put dam-building beavers to shame.

JUERGENS: I mean, the beaver is great, but I think these termites surpass the beaver because of the vast area which is turned into grassland.

JOYCE: Juergens published his discovery in the journal Science, in what may be the only scientific article written with fairy circles in the title.

Christopher Joyce, NPR News.

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