sub·ter·ra·nean 
/ˌsʌbtəˈreɪnijən/ adjective
/ˌsʌbtəˈreɪnijən/ adjective formal 1 : located or living under the surface of the ground
▪ subterranean caverns/termites 2 : existing or working in secret
▪ the subterranean [=underground] world of organized crime
▪ subterranean caverns/termites
▪ the subterranean [=underground] world of organized crime
