subterranean subterranean /səbtɝˈeɪniːən/

adjective

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