Answer:
Deep, deep, do they go. Spreading out as they go. Never needing any air. They are sometimes as fine as hair.
Many people own a copy of me. Without me the world would fall.
I am higher than the king
I am used when you jump off a bridge for fun.
I am the home for feathery animals.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.