Dictionary.net

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

Being below the surface of the earth; as, a subterranean cave; hence, hidden; secret. Also, subterraneous.

Lying under the surface of the earth; embedded at some depth in the earth; concealed beneath the surface of the earth; underground.