The Face on Mars is a modern myth; the result of some photographs of the probe Viking 1. Due to a certain way the sunlight illuminated a rock formation on the surface of Mars, it looked like a face on the pictures. Later, more detailed photographs revealed that it had been an optical illusion. However, some people still believe that the Face on Mars is real.
In Martian Dreams, the Avatar can actually locate the face at 37°N 36°W. However, it is comically small -- just as big as two men -- revealing that the programmers obviously didn't take the whole thing seriously, deciding to mock (or pay tribute to) it this way.