Vril: The Power of the Coming Race by Edward Bulwer-Lytton
Edward Bulwer-Lytton, 1st Baron Lytton - Wikipedia, the free encyclopedia
"Its story of a subterranean race waiting to reclaim the surface of the Earth is an early science fiction theme. The book popularised the Hollow Earth theory and may have inspired Nazi mysticism His term "vril" lent its name to Bovril meat extract" (bovine vril)
You may have heard of the Vril Society – 20th century german occultists – who considered the book to be NON fiction, and may be the reasoning behind Hitler’s fascination with ancient societies and the occult.
Regardless of whether it is fact or fiction it is a reasonably interesting read once you get used to the flowery 19th century use of language.