Definitions
1.AdjectiveBelow the ground; below the surface of the Earth."There is an underground tunnel that takes you across the river."
2.AdjectiveHidden, furtive, secretive."These criminals operate through an underground network."
3.AdjectiveOf 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"
4.AdverbBelow the ground."The tunnel goes underground at this point."
5.AdverbSecretly.
6.NounThe regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).
7.NounSynonym of subway: a railway that is under the ground."London Underground"
8.NounA movement or organization of people who resist political convention."the French underground during World War II"
9.NounA movement or organization of people who resist artistic convention.
10.VerbTo route electricity distribution cables underground.
11.Verbsimple past and past participle of undergrind