the place or region beneath the surface of the ground.
11.
an underground space or passage.
12.
a secret organization fighting the established government or occupation forces:
He fought in the French underground during the Nazi occupation of France.
13.
(often initial capital letter) a movement or group existing outside the establishment and usually reflecting unorthodox, avant-garde, or radical views.
14.
Chiefly British. a subway system.
verb (used with object)
15.
to place beneath the surface of the ground :
to underground utility lines.