The Face on Mars
The Face on Mars is an eroded mesa-like landform in the Cydonia region.
Theories of alien construction
Due to the fact that the first few pictures of the face showed a face like structure due to shadows; Fringe theories have arisen that the face (and surrounding mesas) were built by an alien civilization, and that NASA was covering up the evidence. However, this notion is not accepted my prominent Mars scientists, and higher resolution photos of the face taken by the HiRISE cameras of the Mars Reconnaissance Orbiter show that it is just an ordinary landform.