The Collaborative International Dictionary
Understratum
Understratum \Un"der*stra`tum\, n.; pl. L. Understrata, E. Understratums. The layer, or stratum, of earth on which the mold, or soil, rests; subsoil.
Wiktionary
understratum
n. The layer, or stratum, of earth on which the soil rests; the subsoil.
Usage examples of "understratum".
The understratum was yellowish clay, dense and difficult to dig even when it was sodden.