UNDERGROUNDS

An underground railway, especially for mass transit of people in urban areas.

A train that runs on such an underground railway.

A rapid transit system, regardless of the elevation of its right of way.

An underground walkway, tunnel for pedestrians (called pedestrian underpass in us).

An underground route for pipes, sewers, etc.

Regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).

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

Example: "The french underground during world war ii"

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