The Coming Race (UNABRIDGED)

Edward George Earle Lytton Bulwer-Lytton, 1st Baron Lytton (1803-1873) was an English novelist, poet, playright, and politician. Lord Lytton was a florid, popular writer of his day, who coined such phrases as "the great unwashed", "pursuit of the almighty dollar", "the pen is mightier than the sword", and the infamous incipit "It was a dark and stormy night." . The Coming Race drew heavily on his interest in the occult and contributed to the birth of the science fiction genre.
Unquestionably, its story of a subterranean race of men waiting to reclaim the surface is one of the first science fiction novels. The novel centres on a young, independently wealthy traveler (the narrator), who accidentally finds his way into a subterranean world occupied by beings who seem to resemble angels, who call themselves Vril-ya. The hero soon discovers that they are descendants of an antediluvian civilisation who live in networks of subterranean caverns linked by tunnels.
The narrator suggests that in time, the Vril-ya will run out of habitable spaces underground and will start claiming the surface of the earth, destroying mankind in the process, if necessary. (Summary compiled from Wikipedia)