Explanation:

In 1822 the Bavarian observer Franz von Paula Gruithuisen astonished himself and later the world with his discovery of a regular herringbone pattern of ridges arranged with geometric precision. Gruithuisen immediately interpreted this small landscape (which he called Wallwerk) near Sinus Aestuum as a lunar city. It was an easy conclusion to reach based upon the striking pattern of ridges that he drew, and the fact that his predecessors Herschel, Schroeter and others believed that the Moon was inhabited. Perhaps seeming himself as a Columbus of another new world, Gruithuisen showed his city to kings, philosophers and astronomers. He was so famous that he had his choice of university professorships. But then others started looking carefully at his Wallwerk and dismissed it as a natural landscape, and Gruithuisen was also dismissed as something of a crank. But as I was looking at a lovely image of the Moon by Dominique Dierick, I noticed some hilly terrain west of Fra Mauro had a similar linearity and pattern as the Wallwerk. Is there another lunar city? Is the lunar population expanding and are new cities being constructed? Gosh! Look at the the parallel diagonal lines (mouseover for the imaginatively challenged) just north of the peculiar mountain Bonpland Sigma. I fear that Gruithuisen's old dead critics would be right again if they doubted that this were an artificial landscape. The lighting does enhance the detectability of alignments of mostly buried ridges and hills, but they are outliers of the Imbrium ejecta seen north of Fra Mauro A (10 km wide). Under the higher lighting of the Lunar Orbiter IV view the parallel ridges disappear and - poof! - the lunar city vanishes.

— Chuck Wood

Technical Details:
Image mosaic taken with C9.25, AP Barcon 2x and webcam

More about Gruithuisen: Chapter 7 of Epic Moon by Sheehan & Dobbins

Related Links:
The Twilight Zone of Photography
Lunar Orbiter IV-125 H3
Rukl Atlas of the Moon sheet 42

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