Definition: [adverb] beneath the surface of the ground.
Example: miners working underground
Definition: [adjective] situated beneath the surface of the ground.
Example: an underground car park
Definition: [noun] an underground railway, especially the one in London.
Example: travel chaos on the Underground
Definition: [noun] a group or movement organized secretly to work against an existing regime.
Example: the French underground
Definition: [noun] a group or movement seeking to explore alternative forms of lifestyle or artistic expression.
Example: the late sixties underground
Definition: [verb] lay (cables) below ground level.
Example: sections of electricity line had been undergrounded