underground

1. (not comparable) Below the ground; below the surface of the Earth.Examples: "There is an underground tunnel that takes you across the river."; "One of the hidden glories of Victorian engineering is proper drains. Isolating a city’s effluent and shipping it away in underground sewers has probably saved more lives than any medical procedure except vaccination."Synonyms: subterranean, hypogeannot-comparable

2. (figurative) Hidden, furtive, secretive.Examples: "These criminals operate through an underground network."Synonyms: covered, concealed, dark, dern, hidden, invisible, latent, privatefiguratively

3. (of music, art etc.) Outside the mainstream, especially unofficial and hidden from the authorities.Examples: "underground music"; "‘[…]he wrote to me last week telling me about an incredible bitch of a row blazing there on account of someone having been and gone and produced an unofficial magazine called Raddled, full of obscene libellous Oz-like filth. And what I though, what Sammy and I thought, was—why not?’ ‘Why not what?’ said Tom. ‘Why not do the same thing here?’ ‘You mean an underground magazine?’ ‘Yup.’"; ""In many ways, it showed there was no longer an underground, as such. This proved that there was no longer one society with everyone agreeing how to live . . . The underground had officially come above ground, and consequently no longer existed.""Synonyms: unconventional, alternativeAntonyms: mainstreamusually