Looks like any number of Kettle Lakes left over from the last glacial episode some 15,000 years ago. All of which have been getting filled in with sediment over that time period.
We used to play at one as a kid. It was peat bog all around the edge and you could never get to the actual water as we would get too scared we would fall through the floating peat. We’d take a stick and when it poked all the way through the peat and into the water below we knew we were getting close.
Went back to it a few summers ago after being away for 30 years and you could walk a lot farther out on top of the lake. I suppose in another 100 years the peat will completely cover the water.