Underground meaning

a secret group organized to overthrow a government or occupation force

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'”

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”

in or into hiding or secret operation

“the organization was driven underground”

beneath the surface of the earth

“water flowing underground”