I’m not exactly sure how the round water valve cover came to be surrounded by an oval. It doesn’t look like a machine-made cut. I like this photo for this mystery and that it could be a Ptolemaic schematic in which the earth is the center of everything.
Oval in a Square
I believe this is one of the worldwide chain of symbols that aliens will read when they finally get here, if they bother. …
I believe you are right. Carl Sagen couldn’t have done better if he made it up.