The Knocker, Knacker, Bwca (Welsh), Bucca (Cornish) or Tommyknocker (US) is the Welsh and Cornish equivalent of Irish leprechauns and English and Scottish brownies. About two feet tall and grizzled, but not misshapen, they live beneath the ground. Here they wear tiny versions of standard miner's garb and commit random mischief, such as stealing miner's unattended tools and food.
Their name comes from the knocking on the mine walls that happens just before cave-ins – actually the creaking of earth and timbers before giving way. To some of the miners, the knockers were malevolent spirits and the knocking was the sound of them hammering at walls and supports to cause the cave-in. To others, who saw them as essentially well-meaning practical jokers, the knocking was their way of warning the miners that a life-threatening collapse was imminent.
While some variety of "Little People" was common to all Celtic and northern Germanic peoples, the origin of knockers probably comes from early Welsh mythology, in which they may have been the pre-Brythonic inhabitants of the Celtic west of Britain. Skilled in the arts of mining and tunnelling, they taught these arts to the Britons. These legends may have influenced Tolkien's concept of the Dwarves, consummate miners and stoneworkers who taught these skills to men.
According to some Cornish folklore however, the Knockers were the helpful spirits of people who had died in previous accidents in the many tin mines in the county, warning the miners of impending danger. To give thanks for the warnings, and to avoid future peril the miners cast the last bite of their pasties into the mines for the Knockers.
In the 1820s, immigrant Welsh miners brought tales of the knockers and their theft of unwatched items and warning knocks to western Pennsylvania, when they gravitated there to work in the mines. Cornish miners, much sought after in the years following the 1848 gold rush, brought them to California. When asked if they had relatives back in Cornwall who would come to work the mines, the Cornish miners always said something along the lines of "Well, me cousin Jack over in Cornwall wouldst come, could ye pay ’is boat ride", and so came to be called Cousin Jacks. The Cousin Jacks, as notorious for losing tools as they were for diving out of shafts just before they collapsed, attributed this to their diminutive friends and refused to enter new mines until assured by the management that the knockers were already on duty. Belief in the knockers remained well into the 20th century. When one large mine closed in 1956 and the owners sealed the entrance, fourth, fifth, and sixth generation Cousin Jacks circulated a petition calling on the mineowners to set the knockers free so that they could move on to other mines. The owners complied.