Definition of "underground"

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.

2014 June 14, “It's a gas”, in The Economist‎, volume 411, number 8891

‘ […] 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.’

1991, Stephen Fry, The Liar, page 27

"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."

2010 March 20, James Campbell, “Barry Miles: 'I think of the 60s as a supermarket of ideas. We were looking for new ways to live'”, in The Guardian‎

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

1942, Julius J. Epstein, Philip G. Epstein, Howard Koch, Casablanca, spoken by Ilsa (Ingrid Bergman)

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.

1962, David Pesonen, “Battles Over Energy”, in Carolyn Merchant, editor, Green Versus Gold: Sources in California's Environmental History‎, Island Press, published 1998, page 325

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.

2004, Don L. Ivey, C. Paul Scott, “Solutions”, in Transportation Research Board Committee on Utilities, editor, Utilities and Roadside Safety‎, State of the Art Report 9, Transportation Research Board, page 9

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

2006, Janes Northcote-Green, Robert Wilson, “Design, Construction and Operation of Distribution Systems, MV Networks”, in Control and Automation of Electrical Power Distribution Systems‎, CRC Press, page 110