"The authors acknowledge the use of Mars Orbiter Camera images processed by Malin Space Science Systems that are available at http://www.msss.com/moc_gallery/."

This Viking image shows a demolished square structure on Mars, approximately 4-8 km in size -- monumentally huge.


See anything interesting here?  Tell me it isn't a city.  Let's have a closer look at that square structure:


OK, it's not perfectly square.  The left and right sides are slumped a bit.  If it were a pyramid and fell apart, was bombed, or collapsed, I'd expect more debris to fall down the middle of each side than at the corners.  Also there seems to be some damage to the left corner and bottom side.  A 1-2 fractures and a slump.

The top is gone or caved in, but isn't that some kind of hole or depression toward the left side on the top (the darker area)?

How big is it?  Not sure.   The Viking images were published without scale information.  My best guess is  200 m/pixel and this structure is about 40 pixels across = 8 km???  HUGE.  It could be half that, given my uncertainty about the scale, but even 4 km is huge.