A walled plain flooded with dark lava. With an albedo of 0.06 it is one of the darkest areas of the Moon. Under low illumination, when it is near the terminator, the remains of a fragmented but exceptionally wide outer wall can be seen.
The smaller dark crater to the left (east) is Riccioli.
Photo: António Cidadão. His Home Page of Astrophotography includes stereograms, animations, and "Seven Craters a Week" from his forthcoming photographic lunar atlas.