A subshaft is a small, secondary shaft or tunnel in a mine or quarry.
Subshafts are important for getting materials out of the ground, but they are usually dark and dangerous places.
Examples:
The miners went down the subshaft to get coal.
The subshaft is very dark and scary.
The subshaft is where the miners work.
I don't want to go into the subshaft, it's too dark!
The subshaft goes very deep into the ground.
The subshaft is where the rocks come from.