Investigation of the images returned by Mariner 9 and the Viking 1 & 2 probes have lead some people to believe that there are many large scale artificial structures on Mars.
The most famous structure being "The Face":
Several pyramidal shapes can also be seen:
Other images reveal the "Inca City""
For detailed information about the pictures there are several sites available:
- Views of the Solar System - Face on Mars analysis provides raw images, processed images, information about image processing, and more.
- National Space Science Data Center - Mars page has information on the "Inca City" and Mars Face formations.