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Adjective: subterranean  ,súb-tu'rey-nee-un
  1. Being or operating under the surface of the earth
    "subterranean passages";
    - subterraneous
  2. Lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed)
    "subterranean motives for murder";
    - subterraneous, ulterior

See also: covert, subsurface

Encyclopedia: Subterranean