Subterranean
WordNet
adjective
(1) 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
(2) Being or operating under the surface of the earth
"Subterranean passages"
"A subsurface flow of water"