The Coming Race by Edward Bulwer-Lytton

Edward Bulwer-Lytton was a prominent English writer and politician in the 19th century.  Bulwer-Lytton is notable for being one of the first authors to earn a considerable fortune from just his books.  Bulwer-Lytton also was responsible for famous sayings such as "pursuit of the almighty dollar" and "the pen is mightier than the sword".  Some of his most famous works include The Last Days of Pompeii, The Coming Race, and Zanoni.

The Coming Race, published in 1871, is a novel that centers around a young, unnamed, wealthy traveler who visits a friend.  They explore a mysterious chasm in a mine and the narrator discovers a new, superior race.