say it: suhb-tur-AY-nee-uhn
subterranean is an adjective meaning "being or operating under the surface of the earth". It carries 2 recorded senses.
being or operating under the surface of the earth
“subterranean passages”
“a subsurface flow of water”
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”
Definitions, synonyms and semantic relations for subterranean come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.