un·der·ground
ˌən-dər-ˈgrau̇nd 
1
: beneath the surface of the earth
2
: in or into hiding or secret operation
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
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
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
Kennedy French, Variety, 6 Mar. 2026
Now, less than a year after that Handmaid’s Tale ending where June vowed to continue fighting underground alongside the Mayday resistance group to find Hannah and take down the patriarchal regime, the TV adaptation of The Testaments will debut April 8 on Hulu.
—Max Gao, HollywoodReporter, 5 Mar. 2026
Karim Sadjadpour, The Atlantic, 10 Mar. 2026
Bunn had previously used AI to search for new sources of oil underground, but pivoted to medical work after his wife had a series of miscarriages.
—Meg Wingerter, Denver Post, 4 Mar. 2026
Christopher McFadden, Interesting Engineering, 8 Mar. 2026
These natural gems give a glimpse into the Floridan aquifer, a system of porous limestone that forms underground caves and rivers, pushing water up through vents and ultimately flowing into the faucets of millions of Sunshine State residents.
—Patrick Connolly, The Orlando Sentinel, 7 Mar. 2026
See All Example Sentences for underground
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Adverb
The black marks across Buckley’s face in the film are, in the production’s visual conception, the ink of Shelley’s own manuscript bleeding out — a woman underground for two centuries, unable to finish the thought.
—Kennedy French, Variety, 6 Mar. 2026
Now, less than a year after that Handmaid’s Tale ending where June vowed to continue fighting underground alongside the Mayday resistance group to find Hannah and take down the patriarchal regime, the TV adaptation of The Testaments will debut April 8 on Hulu.
—Max Gao, HollywoodReporter, 5 Mar. 2026
Noun
His father was forged by years of revolutionary hardship—prison, persecution, life underground—before coming to power and reportedly amassing a war chest exceeding $100 billion, built in part from properties confiscated from religious minorities who fled persecution.
—Karim Sadjadpour, The Atlantic, 10 Mar. 2026
Bunn had previously used AI to search for new sources of oil underground, but pivoted to medical work after his wife had a series of miscarriages.
—Meg Wingerter, Denver Post, 4 Mar. 2026
Adjective
Degrading Iran’s war machine The larger munitions would have likely been used to penetrate underground facilities, destroy missile launch bunkers, and collapse tunnels or storage depots.
—Christopher McFadden, Interesting Engineering, 8 Mar. 2026
These natural gems give a glimpse into the Floridan aquifer, a system of porous limestone that forms underground caves and rivers, pushing water up through vents and ultimately flowing into the faucets of millions of Sunshine State residents.
—Patrick Connolly, The Orlando Sentinel, 7 Mar. 2026
See All Example Sentences for underground
Cite this Entry
“Underground.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/underground. Accessed 14 Mar. 2026.
Copy CitationKids 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
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