adj.1. Situated, occurring, or operating below the surface of the earth: underground caverns; underground missile sites.
2. a. Hidden or concealed; clandestine: underground resistance to the tyrant.
b. Of or relating to an organization involved in secret or illegal activity: underground trade in weapons.
3. 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.1. 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).
2. Chiefly British A subway system.
3. An avant-garde movement or publication.