underground

adj

1. (not-comparable)Below the ground; below the surface of the Earth.

   “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.”

2. (figuratively)Hidden, furtive, secretive.

   “These criminals operate through an underground network.”

3. (figuratively)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”

   “‘[…]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."”

adv

1. Below the ground.

   “The tunnel goes underground at this point.”

2. Secretly.

noun

1. The regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).
2. (British)Synonym of subway: a railway that is under the ground.

   “London Underground”

3. (with-definite-article)A movement or organization of people who resist political convention.

   “the French underground during World War II”

   “Victor, please, don't go to the underground meeting tonight.”

4. (with-definite-article)A movement or organization of people who resist artistic convention.

verb

1. To route electricity distribution cables underground.

   “One is to underground where no other alternative will work, and this method should be used universally in urban regions as it now is in “downtown” sections.”

   “Also, undergrounding may not eliminate the potential for crashes with other roadside objects, such as trees, walls, buildings, and so forth. [...] When looking at the fesibility of undergrounding utilities, the complete roadside area and nearby adjacent properties should be evaluated for potential roadside obstructions or hazards.”

   “The utility now wants the network to be undergrounded in the urban areas, which would mean substations with 33 kV distribution swtichgear.”

2. (form-of, participle, past)simple past and past participle of undergrind

name

1. (UK)The London Underground.

   “Although some of the stations on ' The Branch ' have not, as yet, suffered any considerable structural alterations, all are now equipped with the familiar Underground type name boards, and as a result the imposing titles of G.N. days have been ruthlessly curtailed.”

   “Heathrow has a 28% share of passengers arriving by rail, two-thirds of whom come by the Underground.”