This image of Kebira Crater on the NASA Earth Observatory Web site
is from Landsat 7, taken on March 15, 2001:

http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17208

To quote:

But why had no one noticed the giant Kebira crater before? El-Baz speculates that the crater’s massive size hid it in plain view. “The search for craters typically concentrates on small features, especially those that can be identified on the ground. The advantage of a view from space is that it allows us to see regional patterns and the big picture,” he said in a Boston University press release. Also, the double-ringed crater sits in sandstone that is 100 million years old, which means that the impact probably occurred around 100 million years ago. In the intervening time, wind and water have worn features of the crater away, making it hard to identify. For example, the beds of two ancient rivers run from east to west across the crater, leaving two gaps in the inner ring on the upper right side.