"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"