- a secret agent.
- an undercover investigation.
- belowground storage areas.
- clandestine intelligence operations.
- cloak-and-dagger activities behind enemy lines.
- hole-and-corner intrigue.
- in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'.
- secret missions.
- secret sales of arms.
- surreptitious mobilization of troops.
- the organization was driven underground.
- underground caverns.
- underground resistance.
- water flowing underground.