I just finished reading The Moon Pool (1919) by A. Merritt. People say that this influence Lovecraft and the TV series "Lost", and they are right. I found out also that it influenced The Mole People (1956). I'll explain more in a somewhat spoilery fashion behind a cut.
The novel is sometimes claimed to be the inspiration behind "The Call of Cthulhu", but I'd expand that statement to say that it influenced most of Lovecraft's work. I think it's fair to claim that Lovecraft's mission with the so-called Mythos horror stories was to capture the themes and idea presented in The Moon Pool, but re-interpreted as horror instead of romantic adventure. Instead of the partially-sunken city of Nan-Tauach in the South Pacific, we get the once-sunken but now risen city of R'lyeh in the South Pacific. We even get reference to weird angles in both works. The descent into the lost world of Muria echoes descriptions in both "At the Mountains of Madness" and "
The Mound" (ghostwritten for Zealia Bishop.) There are frog-people (the Akka,) reminiscent of the fish-people of "Dagon" and "The Shadow Over Innsmouth". There is an unbelievably ancient, unbelievably advanced race of saurian titans, the Taithu, born near the Earth's heart, which seem to be the pattern for the Old Ones. They have a means of travel (or is it remote viewing?) suggested later in "Dreams in the Witch House". And they created The Shining One, whose eerie, almost musical alien sounds and amorphous nature are echoed in the shoggoths of "At the Mountains of Madness" (they even rebel against their creators, as does The Shining One. Oh, and it's described as a thing of swirling light and mist surmounted by glowing globes, which is reminiscent of the "congeries of lights" used to describe Yog-Sothoth in "The Dunwich Horror".
But when I was reading The Moon Pool, I was also strongly reminded of the '50s film The Mole People. It's not set in the South Pacific, but in the Himilayas -- but it seems that another A. Merritt novel with the same narrator character, The Metal Monster, starts with an expedition to the Himilayas, so maybe that's not a significant difference. There are a lot of trivial changes to the basic story, but enough is recognizable: deep below the Earth is a huge cavern with a lost civilization (Sumerians, instead of Murians.) There's a sharp social divide, with the rulers treating the lower caste as slaves... but one slave-woman inspires love in one of the surface visitors because she has a markedly different appearance, a throw-back to their surface-dwelling ancestors. There's a horrible chamber where political prisoners are sacrificed, but instead of being sacrificed to The Shining One (a creature of living light,) the chamber contains a shaft to the surface that allows horrible, horrible sunlight to burn the flesh of the cave-adapted Sumerians. The technology in the film is much more subdued, lacking the super-science elements of The Moon Pool. In fact, that's a good summary of The Mole People: It's The Moon Pool watered down, with the serial numbers filed off.
The real reason I read The Moon Pool, though, was because I'd heard that "Lost" was inspired by it. It is, in a peculiar fashion. I wouldn't say that "Lost" has anything remotely resembling the same plot, and the bulk of the details are different: there's a suggestion of an ancient civilization in the form of ruins, but nothing like the Murians and no super-science. Desmond in "Lost" seems to be inspired by O'Keefe in The Moon Pool: both are Celtic (Desmond is Scottish, O'Keefe is Irish,) both are ex-military, both have warm personalities that inspire immediate admiration in other characters, both have some supernatural visions, both are castaways, and both long to be united with one true love (although O'Keefe's love is a Murian woman, not a former love interest like Penny.)
But the real clincher is Smokey, the insubstantial killer monster on the island of "Lost". The Shining One (or Dweller in the Moon Pool) is clearly the inspiration for Smokey, even though one is dark smoke and the other light mist. They both make a weird noise, snatch people up into the air, and carry them down below the ground, where there's a magical glowing pool. Oh, and there's an explicit comparison of both to The Devil.
I do recommend The Moon Pool, which is a pretty good read even with its dated language. I feel like it influenced a lot of other fantasy/adventure stories beyond Lovecraft and "Lost".