underground

1 of 3

adverb

un·​der·​ground ˌən-dər-ˈgrau̇nd 

1

: beneath the surface of the earth

2

: in or into hiding or secret operation

underground

2 of 3

noun

ˈən-dər-ˌgrau̇nd 

1

: a subterranean space or channel

2

: an underground city railway system

3

a

: a movement or group organized in strict secrecy among citizens especially in an occupied country for maintaining communications, popular solidarity, and concerted resistive action pending liberation

b

: a clandestine conspiratorial organization set up for revolutionary or other disruptive purposes especially against a civil order

c

: an unofficial, unsanctioned, or illegal but informal movement or group

especially : a usually avant-garde group or movement that functions outside the establishment

underground

3 of 3

adjective

un·​der·​ground ˈən-dər-ˌgrau̇nd 

1

: being, growing, operating, or situated below the surface of the ground

2

: conducted by secret means

3

a

: existing outside the establishment

an underground literary reputation

b

: existing outside the purview of tax collectors or statisticians

the underground economy

4

a

: produced or published outside the establishment especially by the avant-garde

underground movies

underground newspapers

b

: of or relating to the avant-garde underground

an underground moviemaker

an underground theater

Synonyms

Noun

• resistance

Adjective

• clandestine
• undercover
• covert
• sneak
• private

Examples of underground in a Sentence

Adverb

They had been living underground as fugitives.

Noun

I've ridden on the New York subway, the Paris Metro, and the London Underground.

joined the underground while still a teenager

Adjective

The drugs are supplied through an underground network.

She loves the city's underground music scene.

Recent Examples on the Web

Adverb

Boston’s Big Dig, which buried a central highway underground, was infamously plagued by cost overruns but eventually transformed the city’s urban landscape and boosted the economy. —Tina Li, Sacbee.com, 14 Aug. 2025

In frustration, the AEC turned to another Plowshare experiment, Project Gnome, a detonation in New Mexico expected to be fully contained underground. —Shoshi Parks, Popular Science, 14 Aug. 2025

Noun

The speaker system—designed in part by Shepherd and Tom Smith, co-founder of the famed underground Cosmic Slop party in Leeds—was meant to reference the disco setups at New York’s Paradise Garage and sub-heavy sound systems of Jamaica. —Jeremy D. Larson, Pitchfork, 14 Aug. 2025

Her family, which owns the oil and gas underground, received a percentage of the revenue generated from the company’s sale of the minerals, called a royalty. —Jacob Orledge, ProPublica, 4 Aug. 2025

Adjective

Tunneling too close to the river could risk breaching the water table and filling a karst formation, or underground cave — potentially triggering leaks of construction materials into the waterway. —Stuart Dyos, Nashville Tennessean, 14 Aug. 2025

The average build-out cost per mile of fiber for internet service per mile can run as high as $80,000 for underground cable & $40,000 for aerial cable. —PC Magazine, 14 Aug. 2025

Word History

First Known Use

Adverb

14th century, in the meaning defined at sense 1

Noun

1594, in the meaning defined at sense 1

Adjective

1601, in the meaning defined at sense 1

Phrases Containing underground

• go underground
• the underground

Rhymes for underground

• aboveground
• battleground
• belowground
• paperbound
• turnaround
• ultrasound
• wraparound
• abound
• aground
• around
• astound
• background

Kids Definition

underground

1 of 3 adverb

un·​der·​ground ˌən-dər-ˈgrau̇nd 

1

: below the surface of the ground

an underground passage

2

: in or into hiding or secret operation

the political party went underground

underground

2 of 3 noun

un·​der·​ground

ˈən-dər-ˌgrau̇nd

1

: a space under the surface of the ground

especially : subway sense 2

2

: a secret political group

especially : an organized body working in secret to overthrow a government or an occupying power

underground

3 of 3 adjective

un·​der·​ground ˈən-dər-ˌgrau̇nd 

1

: being, growing, operating, or located below the surface of the ground

an underground stream

2

: conducted secretly