underground

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.

Hidden, furtive, secretive.

These criminals operate through an underground network.

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.’

Below the ground.

The tunnel goes underground at this point.

Secretly.

Regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).

Synonym of subway: a railway that is under the ground.

London Underground

A movement or organisation of people who resist political convention.

the French underground during World War II

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

A movement or organisation of people who resist artistic convention.