At first glance I could not breath for a while as spotted this strange thing -with shape of a torus- under the water surface.
It looks as it were the spot where water drains under the bedrock, so thought, probably it can be a
Cover-subsidence sinkhole (?) leading to some kind of an underground reservoir, but how sand-torus has been formed is still a mistery for me.
On image slide below you can see phases of the process:
# Phase1 - Drainage - The torus exists - overlay (
ArcGIS/esri imagery 2011-2014)
# Phase2 - Water has gone, bedrock partly visible (GE imagery 2014)
# Phase3 - Desert sand returned (GE imagery 2018)
[slide];https://res.cloudinary.com/syzygy/image/upload/v1592983088/desert.sinkhole.phase1_b6qso8.jpg,Phase1 - Drainage - The torus exists - overlay (ArcGIS/esri imagery 2011-2014);https://res.cloudinary.com/syzygy/image/upload/v1592983089/desert.sinkhole.phase2_nsly02.jpg,Phase2 - Water has gone, bedrock partly visible (GE imagery 2014);https://res.cloudinary.com/syzygy/image/upload/v1592983088/desert.sinkhole.phase3_nmto1e.jpg,Phase3 - Desert sand returned (GE imagery 2018)[/slide]
Later, on the same day's afternoon when I have spotted it, a strange coincidence heated up my suspicion about this must be some kind of a drain-hole. Accidentally I have spilled some water on a dirty spot of the garden, and where water have found a quicker way to go underneath (an ants' tunnel or something), the torus shape has been formed in miniature:

Of course, maybe it has nothing to do with that desert feature, only interesting similarity could have grabbed my imagination...
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So... any explanations on what is it and how it formed?
Desert sinkhole - Algeria.kmz (1.46 KB)
ArcGIS overlay (2 zoom levels) and placemark sticked on northeastern edge of the feature attached.