English: The unique lunar feature Ina (center), within Lacus Felicitatis, on the moon. The feature was often called the 'D-Caldera' prior to formal naming in 1979. This is Figure 232 of Apollo Over the Moon (NASA SP-362, 1978), which has the following caption:

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 the accompanying stereogram (fig. 233), the many bulbous structures on the floor give it a blisterlike appearance.
The depression is believed to be volcanic, probably a caldera (El-Baz, 1973b). Figure 234 explains the probable sequence of events leading to the formation of this unusual structure.-F.E.-B.