"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"