AS 17-1672 - "D" like "Depression"...

The steep-walled but shallow D-shaped depression near the center of the photograph is apparently a unique feature. It is located in a patch of mare on the foothills of the Montes Haemus, west of Mare Serenitatis. Measured along its straight side, the depression is about 3 Km wide. It is situated atop a very gentle circular dome that appears to be somewhat smoother than the surrounding mare surface. As is more clearly shown in AS 15-9960, the many bulbous structures on the floor give it a blisterlike appearance.

The depression is believed to be volcanic, probably a caldera (...).