I’ve been toying with the idea of layered history lately. In short, this is a situation where my mind is going off on scores of tangents at once with regard to a worldbuild and I’m not sure how everything’s going to settle or pan out. Then this lightning bolt strikes, a seed is planted (mixing metaphors!), and something interesting comes out of it.
The idea of layered history is intrinsic to urban fantasy, regardless of whether there is a hidden magic or public magic world. It is also intrinsic to Lovecraftian horror (ex. Hellboy). And, of course, the realm of conspiracy theories (the Reptilians, ancient astronauts, etc.).
It is the idea that there are multiple layers of understanding history, or rather multiple layers of information about history. What people believe history is versus what it really is.
In other words, in urban fantasy there is always the surface layer of history: real world history, the things we were all, theoretically, taught in school about world history from pre-historic archaeology to the Renaissance to the Boxer Rebellion to both Iraq Wars. For the purposes of this discussion, I’ll refer to that as mundane history.
Beneath the level of mundane history, all urban fantasy and Lovecraftian horror offers a paranormal history. This is the history of the non-humans, the magical elements of the world. In hidden magic worlds, this is a history that normal people know nothing about. In open/public magic worlds, it may or may not be common knowledge. Rowling codified paranormal history as the History of Magic classes at Hogwarts. Charlaine Harris (Southern Vampire/True Blood) kept vampiric history secret from humans, even after vampires went public, even moreso the histories of the weres, shifters, fae, witches, and others who hadn’t gone public. The True Blood writers took things a step further than Harris and introduced a third layer of history known only to a select handful of vampires (the “vampire bible” and Billith line, in all its absurdity).
I’m currently working on an Earth based urban fantasy setting in which there is mundane history and paranormal history. One group also believes that it has figured out and filled in some of the holes in paranormal history. In truth, they know a handful of little things and extrapolated the wrong things from what little they know, so there’s also real history to account for.