What I find interesting about many of the UFO believers here is a nearly universal distrust of science. Not just our practice of science, mind you, but a seeming suspicion of the empirical method of thought. It's fascinating how many people here believe in titanic Martian smokestacks, a subterranian Martian civilization, etc. I'm new in this specific forum (I spend 95% of my time in God&Science) so I haven't seen any proof of these claims, if you have photos, post them. But in the meantime I'll tell you what I think.
Let's assume for the sake of argument that large, odd structures DO exist on Mars. First, there are some pretty weird structures on earth, too, a top-down, low-res view means nothing until a manned expedition is sent. Second, assuming it DID have an intelligent origin (which is unconvincing and jumping to conclusions) how in the world to you come to the opinion that there is a vast underground civilization under the surface, that it still exists, that it routinely flies ships to earth for centuries for some unknown reason, that it has never revealed itself for some unknown reason, and so on.
Why, in all of documented history, have UFO abductees been continuously the most lonely attention seekers? Coincidence? Or maybe it's the aliens covering up their presence by fooling us!
Why have we detected no radio transmissions from Mars? Hey, they must use some kind of technology we haven't developed yet! Or maybe they maintain radio silence to prevent us from knowing they're there.
Why hasn't NASA reported any conclusive evidence on Martian irregularities given that they have the best equipment to do it with? Well, of course they are part of the conspiracy, a bunch of scientists allegedly searching for truth but actually covering it up for some unknown reason. Hey! Or maybe it's because JPL doesn't want to lose funding! If we sent a manned mission to Mars, NASA certainly would fire everyone in JPL, not give them new jobs in other positions, and CERTAINLY not consider unmanned missions to further destination (i.e. Europa.)
Why don't aliens reveal themselves? Well, they must not want us to know they exist. Hey! Maybe it's like the Prime Directive in Star Trek! Wow, that would be so COOL! They hinder their civilization, hide their presence, build their structures underground just for the sake of little-old-us for millenia. Wow.
What has always amazed me about alien theories is how terracentric they are. Earth is always at the forefront of the aliens' plans. They base everything around it. Rather than go do something exciting with a more advanced race, they feel content to experiment with our biology for thousands of years (apparantly they don't learn too fast or else they wouldn't still need to abduct people. A race of retarded subterranian rat-people, perhaps.)
This Earth-As-Center-Of-Universe mentality betrays a very selfish and arrogant mindset.
No matter how much science insists against alien civilization, well, that's just The Man trying to fool me. But I'm too smart for them!
No matter how hard the aliens are to find, well, that's because they value the human race and don't want to interfere with us. I matter to the aliens!
No matter how much evidence goes to disprove Martian civilization, that somehow bolsters alien-believers. No life found? Well, that's just how clever those aliens are.
This Martian Caretaker view is just another way to psychologically give life some meaning. It is a secular religion, and the paralells are amazing. Both require no proof for their beliefs to be considered valid. Both assume that Man is the greatest thing since white bread and that everyone is looking out for us. Both have a suspicion for science, and assault it when it disproves something (the Martian face) but embraces it when it finds something (the potential Martian germ)
Don't you see that in the end, you not actually looking for aliens? All you're looking for is yourself.