under the surface of the ground
operating secretly or often illegally
"in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"
"belowground storage areas"
"underground caverns"
"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"
"the organization was driven underground"
"water flowing underground"
Come we for this from depth of vnder ground?
出自: T. Kyd
Then sprang the happier day from underground.
出自: Tennyson
Andrew had been underground before, he was used to the..Drineffy mines.
出自: A. J. Cronin