A perpendicular shaft sunk to cut the lode at any required depth.(noun)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "underlayer" in a sentence
"The laminate that was in here was floated, though the underlayer was a cheap sheet of foam that looked like packing material."
"And of course, there's that whole "underlayer" possibility when worn with wrap dresses over them."
"She favored full skirts and shorts, although there was a series of scuba-inspired, slim-fit dresses, or for a twist, there was a scuba top paired with a very feminine, more forgiving lilac-net skirt that had an orange underlayer peeking through."