At the mountains of Madness

Dyer and a graduate student named Danforth fly an airplane over the mountains, which they soon realize are the outer walls of a huge, abandoned stone city of cubes and cones, utterly alien compared with any human architecture. Because of their resemblance to creatures of myth mentioned in the Necronomicon, the builders of this lost civilization are dubbed the "Elder Things". By exploring these fantastic structures, the men are able to learn the history of the Elder Things through interpreting their magnificent hieroglyphic murals: The Elder Things first came to Earth shortly after the Moon was pulled loose from the planet and were the creators of life.
They built their cities with the help of "Shoggoths", biological entities created to perform any task, assume any form, and reflect any thought. As more buildings are explored, a fantastic vista opens of the history of races beyond the scope of man's understanding, including the Elder Things' conflicts with the Star-spawn of Cthulhu and the Mi-go, who arrived on Earth some time after the Elder Things themselves.