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.
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.
Below the surface of the earth.
In secret; stealthily.
To situate under the ground: workers undergrounding telephone lines.