Underground has 11 different meanings across 4 categories:
Noun · Adjective · Adverb · Proper Noun
a secret group organized to overthrow a government or occupation force
"The intelligence report revealed that the underground was planning an attack on the occupying army next month."
an electric railway operating below the surface of the ground (usually in a city)
"in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"
Regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).
"The geologist spent weeks mapping the underground network of ancient limestone caves before descending into the dark tunnels below."
In plain English: An underground is a secret group of people who operate outside the law or mainstream society.
"The construction crew started digging the underground to lay new pipes for the neighborhood."
To route electricity distribution cables underground
"The city council voted to move all overhead power lines underground to reduce visual clutter and improve safety."
In plain English: To go underground means to hide yourself from people who are looking for you.
"The city has decided to dig the new subway line underground."
under the level of the ground
"belowground storage areas"
"underground caverns"
conducted with or marked by hidden aims or methods
"clandestine intelligence operations"
"cloak-and-dagger activities behind enemy lines"
"hole-and-corner intrigue"
"secret missions"
"a secret agent"
"secret sales of arms"
"surreptitious mobilization of troops"
"an undercover investigation"
"underground resistance"
Below the ground; below the surface of the Earth.
"The city's new subway line runs deep underground, passing beneath the historic district without disturbing any buildings above."
In plain English: Underground means located beneath the surface of the ground.
Usage: Use underground as an adjective to describe anything physically located beneath the earth's surface, such as subway systems or bunkers. It is often confused with above-ground when contrasting locations, but both terms refer strictly to physical position rather than metaphorical secrecy.
Below the ground.
"The city's new subway system runs entirely underground to avoid traffic congestion on the surface streets."
In plain English: To go underground means to move below the surface of the ground.
"We took the subway to go underground and avoid the hot weather above."
The London Underground.
"After getting lost on the surface, we finally found our way to the right underground station for Central Line."
The word comes from Middle English, where it was formed by combining "under" and "ground." It entered modern usage as a compound term describing anything located beneath the earth's surface.