This mare is the result of lava filling the central portion of a large impact basin. The Altai Scarp, part of the rim of the outer portion, can be seen to the south-west. The partly submerged walled plain on the southern shore is Fracastorius.
Photo: Morio Higashida. The full size original of this and other photographic and CCD images may be seen on his Invitation to the Moon site.