The Coming Race

http://en.wikipedia.org/wiki/Vril

The Coming Race was originally published anonymously in late 1871

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 civilization who live in networks of subterranean caverns
linked by tunnels. There they live in their technologically supported
Utopia, chief among their tools being the "all-permeating fluid"
called "Vril", a latent source of energy which his spiritually
elevated hosts are able to master through training of their will, to a
degree which depends upon their hereditary constitution, giving them
access to an extraordinary force that can be controlled at will. The
powers of the will include the ability to heal, change, and destroy
beings and things—the destructive powers in particular are awesomely
powerful, allowing a few young Vril-ya children to wipe out entire
cities if necessary. 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.