A knocker, knacker, Bwca (Welsh), Bucca (Cornish) or Tommyknocker (US) was a British spirit of the underground, which is thought to have originated in Wales and branch out to other areas such as Cornwall.
According to Cornish folklore the knocker was the helpful spirit of a previous fatality in the tin mines. A knocker is about 2 feet high, has a disproportionately large head, long beard and weathered, wrinkled skin. Their long arms almost touch the ground and they imitate the miner's clothes in dress, and carry such things as pickaxes and lamps.
Miners knew the knockers well but opinions on their intentions differed. Some believed the knocking on the cave walls caused by knockers were attempts to break down the walls, others that the knocks were directions on where to dig and others still that the knocks were warnings of an impending collapse. Knockers were said to be mischievious, at their best they hid tools and took candles, at their worst they set the tunnels on fire.
Other well known variants of knockers are the tommy-knockers, brought over to America by Cornish miners, the Welsh Coblynau and the Wichtlein.
In the 1700 and 1800 Welsh miners brought the tales, stories myths and legends of the knockers over to North America. The tales of the knockers mischief or sometimes warnings in America were still told in the 20th century. Although not original from North America the knockers like or myths traveled to the new world the people who told their stories.
The knockers are helpful warning the of deadly gas or a mine caving. While they are known to help miners they are also known to be mischievous pulling pranks and tricks but never meaning any harm. They were given the name knockers due to the knocks they sent through the mines to warn miners or in some cases cause a cave in. However if anyone angers a knocker they can go from help mischief makers to dangerous creatures who will harm people if provoked. Although the creatures are described as spirits when the tales were brought over to North America the knockers became more like ghosts in the American miner stories. The knockers are also invisible they can only be heard and are rarely seen. But when they make themselves noticed they appear in the form of a goblin.