What is The Lost City of Z in the nonfiction book, The Lost City of Z?

The mythical El Dorado-like kingdom is the main symbol of Fawcett's lifelong obsession. Fawcett believed a complex and sophisticated society he simply called “Z” existed somewhere hidden in the Amazon. Many of Fawcett's contemporary archaeologists and scientists dismissed the claim under the grounds of “environmental determinism," the belief an environment determines the society that sprouts up around it. No city could exist in the inhospitable jungle according to them. Discovering The Lost City of Z became Fawcett's obsession and a validation of his life's work. The author is also fascinated with validating the theory.