The prominent crater Fabricius (just above and to the right of centre) is entirely contained within a much larger structure. This is Janssen - an extremely eroded walled plain. Its irregular walls are interrupted by many smaller, more recent craters, and its floor is riven with rilles. It is thought to be one of the oldest surviving lunar features.
Photo: Richard Evans. The full size original of this and other lunar images, with additional narratives, may be seen at his Lunar Homepage.