beneath the surface of the ground: traveling underground by subway.
2.
in concealment or secrecy; not openly: subversion carried on underground.
–adjective
3.
existing, situated, operating, or taking place beneath the surface of the ground.
4.
used, or for use, underground.
5.
hidden or secret; not open: underground political activities.
6.
published or produced by political or social radicals or nonconformists: an underground newspaper.
7.
avant-garde; experimental: an underground movie.
8.
critical of or attacking the established society or system: underground opinion.
9.
of or for nonconformists; unusual: an underground vegetarian restaurant.
–noun
10.
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.
Situated, occurring, or operating below the surface of the earth: underground caverns; underground missile sites.
Hidden or concealed; clandestine: underground resistance to the tyrant.
Of or relating to an organization involved in secret or illegal activity: underground trade in weapons.
Of or relating to an avant-garde movement or its films, publications, and art, usually privately produced and of special appeal and often concerned with social or artistic experiment.
n.
A clandestine, often nationalist, organization fostering or planning hostile activities against, or the overthrow of, a government in power, such as an occupying military government: "an underground of dissident intellectuals"(Kenneth L. Woodward).
Chiefly British A subway system.
An avant-garde movement or publication.
adv.
(ŭn'dər-ground')
Below the surface of the earth.
In secret; stealthily.
tr.v.
un·der·ground·ed, un·der·ground·ing, un·der·grounds To situate under the ground: workers undergrounding telephone lines.
[adv., uhn-der-ground; adj., n., v. uhn-der-ground] 
) a movement or group existing outside the establishment and usually reflecting unorthodox, avant-garde, or radical views.