Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

subterranean
     adj 1: being or operating under the surface of the earth;
            "subterranean passages"; "a subsurface flow of water"
            [syn: {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: {subterraneous}, {ulterior}]