A Tommyknocker or simply a Knocker or Knacker is a mythical, subterranean, gnome-like creature in Cornish and Devon folklore. One interpretation holds that they are mine-spirits, believed to be the ghosts of the Jews who worked the mines in the 11th and 12th centuries; another view is that they are the spirits of those killed in a mine.

The Cornish described the creature as a little person 2 ft 0 in (0.61 m) tall, with a disproportionately large head, long arms, wrinkled skin, and white whiskers. It wears a tiny version of standard miner's garb and commits random mischief, such as stealing miners' unattended tools and food.

Cornish miners believed that the diminutive Knockers beckoned them toward finding rich veins of tin. As miners changed from independent, family-owned operators to hired laborers for large industrialized companies, there was an increased concern for safety, reflected in the knockers new role. They knocked on the mine walls to warn of impending collapse.

In the 1820s, immigrant Welsh and Cornish miners brought tales of the Tommyknockers and their theft of unwatched items and warning knocks to western Pennsylvania. Cornish miners, much sought after in the years following the gold and silver rushes, brought them to Colorado, Nevada, and California. The underground elves became part of the folklore of miners throughout the American West, not just those of Cornish background.

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