The Knocker, Knacker, or Tommyknocker is a mythical, subterranean,
gnome-like creature in Cornish and
Devon folklore. Its Welsh counterpart is the coblynau. The
name “knockers,” pronounced “knackers,” comes from the knocking on the mine walls that often happens just before cave-ins. Known for stealing miner's tools they can also be benevolent helping to warn miners of impending collapse.