Stratum :: Stratum (n.) A bed of earth or rock of one kind, formed by natural causes, and consisting usually of a series of layers, which form a rock as it lies between beds of other kinds. Also used figuratively..
Substratum :: Substratum (n.) That which is laid or spread under; that which underlies something, as a layer of earth lying under another; specifically (Agric.), the subsoil..
Understratum :: Understratum (n.) The layer, or stratum, of earth on which the mold, or soil, rests; subsoil..