“Vril: The Power of the Coming Race” by Edward Bulwer-Lytton, from Edda, available in the US and Canada from J D Holmes, may be of interest.
“Sir Edward Bulwer Lytton’s cautionary tale of occult super-powers and advanced subterranean cultures have fascinated readers since 1871. Part early science-fiction, part educational tract, part occult romance, Vril keeps spellbinding readers thanks to its wide range of themes and emotions, as well as its thrilling sense of adventure.
A curious man descends into a mountain through a mine and experiences far more than he bargained for. Deep inside the mountain lies a completely different world. Its inhabitants, the Vril-ya, are human-like but physically superior and philosophically more advanced. They live in harmony made possible by their wisdom but also by the powerful and potentially destructive magical energy: Vril.
The impressed yet terrified visitor is allowed to stay and learn more about their ancient and advanced culture, something very few visitors have—it seems that all the previous adventurers have been mercilessly disposed of by the Vril-ya.”