The place or space beneath the surface of the ground; subterranean space.
- 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.