These days Sir Edward Bulwer-Lytton is best known for the opening lines of his 1830 novel, Paul Clifford, than for his later novel, published in 1871. Entitled The Coming Race, this novel tells the story of a wealthy traveler who, in the course of his journeys, finds himself in a vast subterranean realm ruled by a race of angelic humanoid beings who call themselves the Vril-ya.
The last survivors of an even more ancient civilization, the Vril-ya derive their name from Vril, a strange substance that can take many forms and is imbued with remarkable energy. In the hands of the Vril-ya, Vril can be used either to create or to destroy and many objects are crafted from Vril, thereby imbuing them with great powers of lesser types of matter, whether animate or inanimate. In short, Vril is a master substance and the key to the Vril-ya's remarkable underground utopia.
As described in the book, the Vril-ya are, in every way, superior to human beings. Through their mastery of Vril, they can achieve feats that others might consider magic: flight, telepathy, and healing, as well as the ability to destroy entire cities with a minimum of effort. The Vril-ya are a matriarchy, for among them the female is the dominant sex. Under the guidance of its women, the race is slowly expanding and conquering the world beneath the surface of the Earth. For the moment, mankind need not worry about them, but, in time, the narrator surmises, the Vril-ya will turn their attentions elsewhere and the even the mightiest of Man's empires will prove unable to withstand their assault.
Like a lot of 19th century novels, The Coming Race is difficult to read, as its style is somewhat ponderous and its digressions into various obscure topics render it hard to follow at times. Nevertheless, it's an important work in the "lost world" genre, one that proved quite popular in its day and influenced many later authors. So popular did it become that some 19th century occultists adopted many of its ideas, either unaware or unconcerned that it was a fiction. The term "Vril" is now firmly entrenched in the vocabulary of occultism and some even claim -- without much evidence -- the existence of a secret "Vril Society" that was involved in the rise of Nazism in Germany. Interestingly, "Vril" was also used in the late 1800s to describe a variety of medicinal "tonics," "elixirs," and even foods, all based on the use of the term in The Coming Race. One of these survives to this day, Bovril, which is still produced to this day in the United Kingdom.
The Coming Race is in the public domain, so it should be easy to find a copy of it online. I don't necessarily recommend it for its literary value, although it is an interesting read, but for its importance in establishing many of the convention of lost world/hollow world fiction and for its impact on pseudo-science and occultism, both of which have, in turn, affected the development of pulp fantasy, particularly in its formative years during the 1920s and 1930s.