English: Apollo, AS17-1674 (M) (Altitude: 104km): It is common for the circular form of a very old basin to survive even after all basin materials are obliterated. Mare Vaporum, as shown in this south looking oblique photograph, provides a good example. Its average width is about 200 km. Its circular form marks the outer edge of the ancient, deeply buried Vaporum basin. All the terrae surrounding Mare Vaporum are blanketed by massive ejecta of the Imbrium basin, the center of which lies to the north, behind and to the right of the camera. The ejecta disappears beneath Mare Vaporum. The circularity is enhanced at the left (east) of the picture by a system of mare ridges and scarps that was localized over an old Vaporum basin ring. The cratered, linear Hyginus Rille is near the southern horizon, and the sinuous Conon Rille, to be described later in this book, is in the foreground. (Figure 35 in: H. Masursky, G.W. Colton, and F. El-Baz (eds.) (1978). Apollo over the moon: A view from orbit. NASA, Washington DC)