(UK, rail transport) The London Underground.

(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.

(with definite article) A movement or organisation of people who resist political convention.

(with definite article) A movement or organisation of people who resist artistic convention.

To route electricity distribution cables underground.

(not comparable) Below the ground; below the surface of the Earth.

(figurative) Hidden, furtive, secretive.

(of music, art etc.) Outside the mainstream, especially unofficial and hidden from the authorities.

Below the ground.

Secretly.