The Abyss/Headscratchers

All over the place I see people referring to the undersea creatures as "aliens", when it's never even suggested that they're not from Earth. B'sides, the idea of a more advanced sapient species from our own planet is way more interesting.
 * Apparently the book describes them as aliens. A terrestrial advanced race is cool, true, but not the case here.
 * They're referred to as "non-terrestrial" several times in the movie. Admittedly, the characters have no good reason to make this assumption, but it's not just something the fans made up.
 * Maybe "terrestrial" in this case refers to land-dwelling species?