Meaning : Being or operating under the surface of the earth.
Example :
Subterranean passages.
A subsurface flow of water.
Synonyms : subterraneous
Meaning : Lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed).
Example :
Subterranean motives for murder.
Looked too closely for an ulterior purpose in all knowledge.
Synonyms : subterraneous, ulterior