English: Ina crater, an Irregular Mare Patch (IMP) on the moon. This image was used in figure 25-80 of the Apollo 15 Preliminary Science Report (NASA SP-289, 1972), which has the following caption:
Unusual mare feature of latitude 18°40' N, longitude 5°20' E. Feature may be viewed stereoscopically in these photographs (pan AS15-10176 and 10181).
The text of the Report is Chapter 25. Orbital-Science Investigations, PART N, AN UNUSUAL MARE FEATURE by Ewen A. Whitaker, which states:

While the Apollo 15 panoramic camera photography was being rapidly scanned, a most peculiar feature was noted that presented a totally different appearance from anything seen in all the rest of the Lunar Orbiter and Apollo photography. The feature was missed on Lunar Orbiter IV frame H-1 02 because it is situated in a group of bimat marks. The feature is located at latitude 18°40' N, longitude 5°20' E in a small patch of mare material lying between the Haemus and the Apennine Mountains. This patch is abnormal in that it is an unbordered plateau ; the surface appears to lie several hundred meters above adjacent mare patches.

The feature is D-shaped with a 3-km-long straight edge. Viewed stereoscopically, it is seen to lie perhaps a few tens of meters below the level of the surrounding mare, the latter presenting a convex meniscus at the line of contact. About half of the floor is covered with blobs of mare-like material, reminiscent of dirty mercury. Contacts between the floor and both the mare and blobs are frequently outlined with highly reflective material, perhaps sublimates. The floor also displays some darker areas that have noticeably different photometric properties from the maw surface and the blobs. The whole feature is seen to be almost devoid of small impact craters, thus differing from the surrounding mare. Two panoramic camera photographs of the feature are shown in figure 25-80, which may be viewed stereoscopically.