Definitions of subterranean

1. 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

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

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