Very fantastic imagination and description by daniken. When I first time red this book 15 years ago I was fascinated by this book and amazing places & secrets he describes in his book. He has a theory that all of those ancient secrets and ancient human made structures & monuments are guided or constructed by intelligent aliens.
Today I have more than 10 books of him all read. not because of his theory but for the best description of the secrets on earth and human past.
As richard feynman Noble Prize winner once said in one of his speeches about UFO:
"Listen, I mean that from my knowledge of the world that I see around me, I think that it is much more likely that the reports of flying saucers are the results of the known irrational characteristics of terrestrial intelligence than of the unknown rational efforts of extra-terrestrial intelligence. It is just more likely. That is all"
As same as he said it is more likely these secrets have been made by human ancestors who have been fallen out in the past than by aliens. Such as atlantis legend.
If one read some of the books about finding extraterrestrial life he would understand how rare it is in this vast unlimited universe. And I think his theory is exaggerated.