subsoil subsoil /ˈsʌbˌsɔɪl/

noun

1. () The layer of soil between the topsoil and bedrock  ( undersoil , subsoil )
   the layer of soil between the topsoil and bedrock
    I dug down to the subsoil layer to improve drainage in my garden beds
   I dug down to the subsoil layer to improve drainage in my garden beds