SUBTERRANEAN
subterranean, subterraneous, ulterior
(adjective) 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
subterranean, subterraneous
(adjective) being or operating under the surface of the earth; “subterranean passages”; “a subsurface flow of water”
Source: WordNet® 3.1
Etymology
Adjective
subterranean (comparative more subterranean, superlative most subterranean)
below ground, under the earth, underground
Synonyms
• subterraneous, subterrene, underground, hypogean
Source: Wiktionary
Sub`ter*ra"ne*an, Sub`ter*ra"ne*ous, a. Etym: [L. subterraneus; sub
under + terra earth. See Terrace.]
Definition: 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: Webster’s Unabridged Dictionary 1913 Edition