Examples from the Web for sub-level
He left the cage at the sub-level just above the last level.
They knew the place as well as though the sub-level had been ablaze with light.
Come on up to the sub-level; we will begin our investigation there.
The men picked up the lad and bore him through the sub-level.
Superintendent Penton grasped the rope that had been made fast to a shore post on the sub-level and let himself down.