Pre-Lovecraftian tales in which the protagonist discovers that the world as we know it is not as it seems, that we're surrounded by ancient, alien, advanced intelligences who watch us, control us, destroy us... that sort of thing.
HOUSE ON THE BORDERLAND
* A. Merritt, "The People of the Pit" (All-Story, January 5, 1918). See Merritt entry.
* A. Merritt, "The Moon Pool" (All-Story, June 22, 1918). See Merritt entry.