The Knockerhood
For as long as men have gone down into mines, they have told stories about mining spirits. Sometimes the spirits were considered malevolent, but more often — particularly in the British Isles — they were essentially seen as being benevolent entities who would warn human miners of dangerous conditions. The Cornish and Welsh called them Knockers, or Bucca; and, like all good tale-telling traditionalists, the miners brought the idea of the Knockers along when immigrating to America.
Where the Knockers promptly joined the union.
The Knockerhood is one of those folk legends that folk only tell after they’ve taken a good look around to make sure that nobody who shouldn’t hear the story does hear the story. This very much includes most folklorists and academics, not to mention nosy government bureaucrats. But some of the older folks will, if genially pressed, admit that the spirits of the mines are card-carrying members of a spectral labor union dedicated to protecting and warning human miners. Or at least the old folks will pointedly not deny it.
The old folks will also admit — or not deny — that people are not entirely certain what the Knockers need a labor union for, given that Knockers ‘work’ for free and don’t have any benefits packages. This is where it’s a shame that spiritualist occultists don’t ever hear about the Knockerhood, because they’d be able to clear the mystery up. The Knockerhood exists in its current form because the Knockers draw their self-image from the expectations and assumptions of humanity. Human miners have labor unions; therefore, so must the Knockers. Those same occultists would also point out that spirits like Knockers cannot really exist without human belief. No more, say, coal miners, no more Knockers — oh, sure, unformed spirits would probably still be around, but they’d have no sentience or personality.
The most immediate adventure hook for a modern campaign is thus hopefully sufficiently obvious that I won’t have to spell it out.