Been reading "At the Mountains of Madness" lately. Already had read "The Color out of Space", "Dream Quest of Unknown Kadath", "The Other Gods" at various times, but this is good solid canonical Lovecraft. It's hitting me how much it isn't about rattling off Ancient Betentacled Horror #1, and Eldritch Gaping Maw #2, and so on --- which is kind of the way it comes down into some of its more casual descendants, who just kind of make a mascot of Cthulhu --- but about discovering there's a mindbogglingly larger, deeper, weirder history of earth than we knew about previously.
Which kind of makes sense for a person living around the beginning of the 20th century, doesn't it? Science had been doing a whole lot of upending of comparatively short, tidy, theologically grounded cosmogonies and replacing them with cold and unsettling narratives of yawningly huge expanses of time and space.
So all you gotta do is populate those yawningly huge expanses with some heretofore unknown civilizations of brain-achingly advanced tentacle beasties that think of humans as pestersome insects at best, and bam, Lovecraftian atmosphere.