Welsh miners moved to western Pennsylvania in the 1820s. As with any group that moves to a new land, they took their stories of the knockers with them. The miners told their new co-workers tales of stolen tools and helpful warnings.

Cornish miners took the legends to California. Their mining abilities were so sought after that mine owners even paid the boat fare to bring more Cornish miners to the area. Yet the new workers brought demands of their own. They wouldn’t enter new mines unless managers assured them the helpful spirits were present.

Source: Who are the mysterious Knockers of Cornish folklore?