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.