Meaning

• 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.

Meaning

• 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.

  "the French underground during World War II"

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

Meaning

• To route electricity distribution cables underground

Meaning

• Below the ground; below the surface of the Earth.

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

• Hidden, furtive, secretive.

  "These criminals operate through an underground network."

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

  "underground music"

Meaning

• Below the ground.

  "The tunnel goes underground at this point."

• Secretly.