underground

Situated, occurring, or operating below the surface of the earth.

Hidden or concealed; clandestine.

Of or relating to an organization involved in secret or illegal activity.

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.