beneath the surface of the ground

traveling underground by subway.

in concealment or secrecy; not openly

subversion carried on underground.

existing, situated, operating, or taking place beneath the surface of the ground.

used, or for use, underground.

hidden or secret; not open

underground political activities.

published or produced by political or social radicals or nonconformists

an underground newspaper.

avant-garde; experimental

an underground movie.

critical of or attacking the established society or system

underground opinion.

of or for nonconformists; unusual

an underground vegetarian restaurant.

the place or region beneath the surface of the ground.

an underground space or passage.

a secret organization fighting the established government or occupation forces

He fought in the French underground during the Nazi occupation of France.

( often initial capital letter ) a movement or group existing outside the establishment and usually reflecting unorthodox, avant-garde, or radical views.

Chiefly British . a subway system.

to place beneath the surface of the ground

to underground utility lines.

adjective

aboveground , ground , sky , authorized , condoned , known , legal , public

adjective

below ground , buried , covered , in the recesses , subterranean , subterrestrial , sunken , underfoot , alternative , avant-garde , clandestine , concealed , covert , experimental , hidden , hush-hush * , on the qt , on the sly , private , radical , resistant , resistive , revolutionary , surreptitious , unbowed , unconventional , undercover , under wraps * , unusual , hypogeal , hypogean , hypogeous , beneath , chthonian , illegal , secret , submundane , subterraneous , subterrene , subversive , subway