The Treasure Keeper (in Polish "Skarbnik") is the legendary spirit of the mine.
In Polish mythology they live in mines and underground workings and are the guardians of gems, crystals, and precious metals.
In Wieliczka salt, of course, is the treasure in question.
Skarbnik is a good spirit, but he always comes bearing a warning. If he should appear, that means there is dangerous gas, or a cave-in up ahead.
Skarbnik will protect miners from danger, and lead them back when they are lost.