The Knocker, Knacker, Bwca (Welsh), Bucca (Cornish) or Tommyknocker (US) is a mythical creature in Welsh, Cornish and Devon folklore. They are the 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 miners' 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.
According to some Cornish folklore, 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 tasty pasties into the mines for the Knockers.
A South African author, Stuart Cloete, wrote about the Knockers in 1942 as being friendly unless crossed or teased by the miners. They were smaller than a yearling child with big heads and the faces of old men. They were originally the spirits of the Jews who voted to crucify Jesus, and who were too bad for Heaven, and too good for Hell, so they were remained on the face of the earth where their presence in mines and tunnels usually indicated good ore.
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.
Knocker also appeared as a name for the same phenomena, in the folklore of Staffordshire miners.
Tommyknockers are also a motif found in a science fiction/horror book by Stephen King.