Knockers live below ground and look like little miners. They knock on the walls of mines. They are Welsh and Cornish in origin, and some see them as mischief-makers where others consider them harmless practical jokers. English Brownies and Irish Leprechauns are comparable.
Blue-caps are similar to Knockers, but they are benevolent and forwarn people about cave-ins. They show miners where rich deposits can be found, as long as they are treated with respect.