undersoil

Noun

undersoil (plural undersoils)

1. The soil underneath the surface / topsoil and above the bedrock.
   • 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 […]1845, Charles Darwin, Journal of Researches into the Natural History and Geology of the Countries Visited during the Voyage of the H.M.S. Beagle Round the World, 2nd edition, London: John Murray...
   • 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.1916, D. H. Lawrence, “Discipline”, in Amores, London: Duckworth, page 37