Underwater ring of sand - Algeria

[syzygy]

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...

***

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.

[leong]

artesian water bore ? The flow  creates the shape  in the sand, and the lake.

then its dry

I see Algeria has capped a number of useless bores in order to protect the supply to required bored.

[syzygy]

I do not think this is a source of water (periodically positive(?) artesian well) , but a depression, where water gathers (and drains beneath). 

My first thought was an entrance of an underground cave, buried by sand - the treasure cave of Aladdin of course...! ( :

Thanks for thought provoking response!