WAVES IN THE DESERT
In a wide area on both sides of the border between Mali and Algeria there are strange large rings whose enclosure is made up of many individual small semicircles. This gives the outer edge an undulating appearance that almost reminds me of grandma's doily. The diameter of these rings is usually 200 to 300 m, while the small semicircles that make them up are about 10 to 15 m in diameter. 
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Images courtesy of Google Earth™
Here, too, I have not yet been able to find out what purpose these structures served, or from what period they date. Were they protective walls around villages? Their size would be quite sufficient for a small village. But they could also be used as enclosures for large herds of cattle. For both, however, the complicated construction of half circles does not make sense at first sight. This purpose could also be achieved with a straight wall, which is easier and faster to build.
These pictures are only a small selection. All the objects I found are included in the following download (Google Earth required): Download placemarks of 55 halfcircle rings >