1. (n.) One who, or that which, underlays or is underlaid; a lower layer.

2. (n.) A perpendicular shaft sunk to cut the lode at any required depth.

band bed bedding bedrock belly belt bottom bottom side breech buttocks couche course deck downside floor fundament gallery hardpan layer ledge level lower side lowest layer lowest level measures nether side nethermost level overlayer overstory rock bottom seam shelf stage step story stratum substratum superstratum thickness tier topsoil underbelly underneath underside understory understratum zone