(not comparable) Below the ground; below the surface of the Earth. / (figuratively) Hidden, furtive, secretive.

Below the ground. / Secretly.

(geography) Regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines). / (chiefly Britain) Synonym of subway: a railway that is under the ground.

To route electricity distribution cables underground.

(UK, rail transport) The London Underground.

plural of underground

Third-person singular simple present indicative form of underground

plural of undergrounder

An underground publication or movie. / One who is part of a secret or underground society or subculture.

simple past tense and past participle of underground