subterranean

(s) being or operating under the surface of the earth
- subterranean passages
- a subsurface flow of water
subterraneous,
(s) 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
ulterior, subterraneous,

subterraneous ulterior