About two feet tall, and often described as greenish in color, they look like men and are most often spied wearing a traditional miner’s outfit. Living beneath the ground, they have been "known” to have committed both good and bad deeds through the centuries, often playing practical jokes and committing random acts of mischief, such as stealing unattended tools and food.

The Tommyknockers were first heard of in the United States when Cornish miners worked in the western Pennsylvania coal mines in the 1820’s. When the California Gold Rush began, these experienced Cornish miners were welcomed and often sought after by the mine owners. Attempting to recruit more minders, managers often approached the immigrants, asking if they had any relatives back in England who might come to work the mines. The Cornish miners would reply something like this: "Well, me cousin Jack over in Cornwall wouldst come could ye pay ’is boat ride." Soon, these many immigrant miners took on the nickname Cousin Jacks, who formed the core of America’s early western mining workforce. As such, their superstition of the Tommyknocker thrived and spread throughout the mines of the west.

The name "knockers,” pronounced "knackers,” comes from the knocking on the mine walls that often happens just before cave-ins. Actually caused by the creaking of earth and timbers, some thought these sounds of "hammering” were malevolent, indicating certain death or injury, while others saw their "knocking” as well-meaning, warning the miners that a life-threatening collapse was imminent. Yet, others believed that the knocking sounds would lead them to a rich ore body and or signs of good luck.

When these grizzled little gnomes were good, they were thought to bring miners favors and wealth. But when they were bad, they were said to bring about misery, injury, and death to those who doubted their power or who did not believe in them.

These highly spirited characters were also known to perform many of the mining duties, working right along side the men, as well as being blamed for many a prank, and credited with saving the lives of many miners. If a hammer was missing, it was the Tommyknockers who had taken it, but if a miner escaped a collapse, the Tommyknockers were given credit.

Later, the legend of the Tommyknockers evolved into the idea that the knockings were caused by dead miners who were kind enough to give warnings of danger to the living. In praise of these kind gestures, the miners would leave offerings of food and other items in order to secure their good graces and protection.