I think they are gold/uranium/diamond mines. these minerals are usually found in channels. depending on the concentration of the minerals, they dug out the sand/mud and half-filled them with chalk, so that they can take an aerial picture and analyze the area for coverage, further prospecting etc.
Some articles speculated they were nuclear testings sites. In one picture you can see what looks like an explosion with surrounding fallout damage. However there are quite a few different structures so who knows.
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