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

Drawing from real events, Macri’s co-production follows Carlita, the first woman miner in a town where superstition barred female presence underground. —Callum McLennan, Variety, 22 Sep. 2025

In addition, by placing the reactor deep underground, the containment structure is no longer required and security is provided by the location of the reactor. —New Atlas, 21 Sep. 2025

Noun

Burnham Yard also is near an Environmental Protection Agency Superfund site for radium contamination, but the Terracon report did not find any indication that there is a substantial amount of radioactive material underground. —Elliott Wenzler, Denver Post, 14 Sep. 2025

But now European and American scientists are exploring whether the first human Martians could find refuge via dormant volcanos scattered across the Red Planet, inside lava tubes that criss-cross the Mars underground. —Kevin Holden Platt, Forbes.com, 9 Sep. 2025

Adjective

Their underground campaign ditched the standard teaser, trailer, poster and TV spots approach for a single cryptic billboard, a mysterious telephone number, and a trail of online breadcrumbs to entice fans to figure out the mystery of the film. —Scott Roxborough, HollywoodReporter, 28 Sep. 2025

For instance, portable cold atom systems could provide navigation without GPS, detect underground structures, monitor climate change with unprecedented detail, and even enable space missions to test gravity and search for new particles. —Rupendra Brahambhatt, Interesting Engineering, 27 Sep. 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