adjective Below the ground; below the surface of the Earth.
There is an underground tunnel that takes you across the river.
adjective Hidden, furtive, secretive.
These criminals operate through an underground network.
adjective Of or relating to an art forms (such as music) or subculture that is outside the mainstream, especially one that is unofficial and hidden from the authorities.
underground music
adverb Below the ground.
The tunnel goes underground at this point.
adverb Secretly.
noun The regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).
noun Synonym of subway: a railway that is under the ground.
London Underground
noun A movement or organization of people who resist political convention.