Today's Image of Mars shows Inca City, a set of intersecting ridges forming rectangular shapes in the South Polar Region of Mars. These ridges are part of a larger circular structure that is approximately 86km in diameter. While conspiracy theorists might believe this really was a city, scientists have more plausible, scientific explanations for why how these formations came to be.
Inca City was likely formed when an ancient impact crater was filled in and covered, only to later become exhumed. Cracks that existed in the crater were filled in with the rest of it and what we see here could be those reexposed fractures. This could happen if the material that covered the crater was less wear-resistant than the underlying surface. This is the exact opposite of how an inverted riverbed forms. While these explanations seem likely, scientists are not certain what created the features you see below.
This image was taken by Mariner 9 in 1972. Clicking on the image will take you to the Mars Global Surveyor Page for the image.