underground line, specifically an underground cable or wire

basement, cellar (floor below ground level)

dungeon

(AmE) subway; (BrE) underground; (BrE) tube

department store's below-ground-level food section

(disaster prevention) Short for 木造住宅密集地域 (mokuzō jūtaku misshū chiiki, “densely built wooden house area”).

birds of a feather flock together

eating food prepared from the kettle in the underworld; it is believed that once this food is eaten, the dead may never return to the land of the living

underground nuclear test

underground passageway