Caption: "What on Earth is that? The Richat Structure in the Sahara Desert of Mauritania is easily visible from space because it is nearly 50 kilometers across. Once thought to be an impact crater, the Richat Structure's flat middle and lack of shock-altered rock indicates otherwise. The possibility that the Richat Structure was formed by a volcanic eruption also seems improbable because of the lack of a dome of igneous or volcanic rock. Rather, the layered sedimentary rock of the Richat structure is now thought by many to have been caused by uplifted rock sculpted by erosion. Why the Richat Structure is nearly circular remains a mystery."
Another picture and caption:
Caption excerpt: "Initially interpreted as a meteorite impact structure because of its high degree of circularity, it is now thought to be merely a symmetrical uplift (circular anticline) that has been laid bare by erosion. Paleozoic quartzites form the resistant beds outlining the structure."
So perhaps my wording was inaccurate; they're attributing the circularity in that part of Lake of the Woods to a "symmetric circular uplift", similar to what formed the Richat Structure. Uplift is what makes mountains; it's unusual to see a circular version.
Whatever it is, it kinda makes your eyes spin when you look at it.