Dictionary

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).

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.

To route electricity distribution cables underground

Below the ground; below the surface of the Earth.

Example: There is an underground tunnel that takes you across the river.

Hidden, furtive, secretive.

Example: These criminals operate through an underground network.

Outside the mainstream, especially unofficial and hidden from the authorities.

Example: underground music

Below the ground.

Example: The tunnel goes underground at this point.

Secretly.