Answer:
I dig out tiny caves and store gold and silver in them. I also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs my help. Yet many people are afraid to let me help them.
Walk on the living, they don’t even mumble, Walk on the dead, they mutter and grumble. What are they?
So cold, damp and dark this place. To stay you would refrain, yet those who occupy this place do never complain.
A word I know, six letters it contains. Subtract just one and twelve remains.
Big as a biscuit, deep as a cup, but even a river can’t fill it up. What is it?
My days are numbered. What am I?