undersoil

1. (geology) The soil underneath the surface / topsoil and above the bedrock.Examples: "It is a singular fact, that on the two great continents in the northern hemisphere […] we have the zone of perpetually frozen under-soil in a low latitude […]"; "But comfort me, my love, now the fires are low, Now I am broken to earth like a winter destroyed, and all Myself but a knowledge of roots, of roots in the dark that throw A net on the undersoil, which lies passive beneath their thrall."; "The childish levels, the undersoil of the mind, had been turned up. She wanted to be with Nice people, away from Nasty people—that nursery distinction seeming at the moment more important than any later categories of Good and Bad or Friend and Enemy."