Synonyms: ulterior, subterranean, subterraneous
Definition: lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed)
Usage: subterranean motives for murder; looked too closely for an ulterior purpose in all knowledge- Bertrand Russell
Similar words: covert
Definition: secret or hidden; not openly practiced or engaged in or shown or avowed
Usage: covert actions by the CIA; covert funding for the rebels
Synonyms: subterranean, subterraneous
Definition: being or operating under the surface of the earth
Usage: subterranean passages; a subsurface flow of water