This guy makes a great case but he does say that it not enough evidence to be absolutely positive of what we are looking at given the current state of our technology today.
There are some pretty strange things on Mars than anyone can check out and see for themselves. I'm here to get the message out there. Feel free to download Google Earth 5.0 (which allows you to explore Mars), copy & paste the coordinates below and check these out for yourself.
Five different entrances on the surface of Mars run in a straight line and go north to south. There is no question that they appear to either be man made or made by intelligent alien life.