Did you see the Ancient Aliens thing on the History Channel again?
Anyway, I do not fully doubt their existence, but I think it's different then what the common person jumps to think.
For instance, (after viewing the program mentioned above and other research in my younger, curious days), I think that the pyramids and other landmarks found in South America didn't just happen. The fact that the pyramids in Egypt perfectly line up to the lattitude and longitude mark in a flawless diagonal line isn't a coincedence. And the fact that the way the pyramids were made are still a mystery.
The thing that I believe more though, is Supernatural activity mistaken for Aliens. For instance, some state that the prophet Enoch predicted the pyramid's construction. The books of Enoch were cut out of the bible, yet they depict Lilith and the Angels explicitly. Lots of primitive drawing and carvings of these 'aliens' are depicted with a halo, and this is before Christianity.
Based on what I believe, I think it also possibly be evidence of activity that is indeed out of this world, but possibly into the realms of Heaven and Hell.