A narrow stretch of relatively flat mare surface extending southwards from Mare Tranquillitatis, the Sea of Tranquility. The mouth of this 'bay' lies on the southwestern edge of Mare Tranquillitatis, and from there it runs for southward (and a little westward) for more than two hundred kilometres. The surface of Sinus Asperitatis shows various ridges, but few craters, with its most significant interior crater being Toricelli, some 20km in diameter.
To the east and west, this strip is bounded by much more broken and heavily cratered terrain. To the south, Sinus Asperitatis approaches the small sea of Mare Nectaris, the Sea of Nectar, and its southern end is marked by two large and prominent craters. These are Theophilus and Cyrillus, each about one hundred kilometres in diameter, and so close that the rim of Theophilus overlaps the crater wall of Cyrillus.