I've always felt that if they ever did find Atlantis - they'd find an alien element. Of course, if it really did exist as explained, I would think that it was an alien ship.
I feel the same about Planet X: if it exists - it's a ship.
Although I do believe in alien life, I think it would have evolved in environments different to ours, and that we would not be recognizable as life to each other. I don't think alien life would have eyes, mouth, nose, legs, arms etc, nor even similar "body-ness". They might not even have need for food, transportation and shelter, as we know it.
Mankind imagines alien life, in an image similar to his own, just as he imagines "God".