adj. Situated, occurring, or operating below the surface of the earth: underground caverns; underground missile sites. adj. Hidden or concealed; clandestine: underground resistance to the tyrant. adj. Of or relating to an organization involved in secret or illegal activity: underground trade in weapons. adj. 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). n. Chiefly British A subway system.n. An avant-garde movement or publication.ad. Below the surface of the earth.ad. In secret; stealthily.v. To situate under the ground: workers undergrounding telephone lines.