I'll try to make this brief. I was looking on a local satellite map in my area, Thumb of Michigan, USA, and I found several circles, all in the area, if not the same section.

Some of it's state land, so I hiked out there to see what it was. There's no trails (new or old distinguishable) to any of these. They're a section of land, about 300-400 feet round, with...20-30 feet of water, like a moat, around it. There's no markings or telling of equipment that I could see that made these. I've asked people who know about the CCC, local historians, soil testing people, and a few logging gurus, and none of them can tell me what it is.

As best as I can tell, the trees are 70-135 years old on these patches of land.

Here's a picture from Imgur https://imgur.com/a/aRt3C0N

And I found it originally from here: https://www.tuscolacounty.org/gis/

You can see a view on Google Maps, but it must have to do with leaves covering it or what not. On the GIS map, there's 20+ of these.