understratum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A substratum.

Century Dictionary and Cyclopedia

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

Wiktionary

1. n. The layer, or stratum, of earth on which the soil rests; the subsoil.

GNU Webster's 1913

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