This prominent walled plain has evidently had a complex history. After its original formation and subsequent flooding, later geological activity has created a network of rilles on its floor, the Rimae Posidonius. The extreme terracing of its south-eastern wall gives the impression of a concentric crater.
Photo: Thierry Legault. The full size original of this and other high definition lunar images may be seen at his High Resolution CCD Imaging site.