understratum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A substratum.

from The Century Dictionary.

• noun A substratum; an underlying stratum; the stratum lying immediately beneath, or forming the lower portion of the one designated: not often used except figuratively.

from the GNU version of the Collaborative International Dictionary of English.

• noun The layer, or stratum, of earth on which the mold, or soil, rests; subsoil.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The layer, or stratum, of earth on which the soil rests; the subsoil.