Ptolemaeus, Alphonsus and Arzachel . ... Credit Copyright: Alan Friedman Explanation: These three ancient, large impact craters lie on the north eastern shores of Mare Nubium, the lunar Sea of Clouds. ... The picture offers a remarkably detailed view of the well-studied region with shadows emphasizing the large crater central peaks and slumping walls. Careful examination also reveals the 110 kilometer long Straight Wall , a fault 200-300 meters high, and the intriguing Davy crater chain . ...