The Collaborative International Dictionary of English v.0.48:

Subterranean \Sub`ter*ra"ne*an\, Subterraneous
\Sub`ter*ra"ne*ous\, a. [L. subterraneus; sub under + terra
   earth. See Terrace.]
   Being or lying under the surface of the earth; situated
   within the earth, or under ground; as, subterranean springs;
   a subterraneous passage. -- Sub`ter*ra"ne*ous*ly, adv.
   [1913 Webster]