subterraneous

Adjective

• 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 or operating under the surface of the earth
  • subterranean passages
    
  • a subsurface flow of water
    

Related words

• Synonyms: 
  subterranean , ulterior