Subterranean

from WordNet (r) 3.0 (2006)

subterranean
    adj 1: being or operating under the surface of the earth;
           "subterranean passages"; "a subsurface flow of water"
           [syn: {subterranean}, {subterraneous}]
    2: lying beyond what is openly revealed or avowed (especially
       being kept in the background or deliberately concealed);
       "subterranean motives for murder"; "looked too closely for an
       ulterior purpose in all knowledge"- Bertrand Russell [syn:
       {subterranean}, {subterraneous}, {ulterior}]

from 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]