Wow. The Manicouagan impact crater is huge. Apparently it’s one of the largest impact craters still preserved on the surface of the Earth, and was formed during a tremendous impact about 200 million years ago. The annular lake that shows the perimeter is 70 kilometers (43 miles) in diameter. Solarviews.com says:

Although the original rim has been removed, the distribution of shock metamorphic effects and morphological comparisons with other impact structures indicates an original rim diameter of approximately 100 kilometers (62 miles).

Thanks to Mike Scher, Philippe, Keith, Stuart Reid, Andy M, Markus and many others.