1. resistance, underground

a secret group organized to overthrow a government or occupation force

• resistance

2. metro, subway, subway system, tube, underground

an electric railway operating below the surface of the ground (usually in a city)

Example Sentences:
'in Paris the subway system is called the metro' and in London it is called the tube' or the underground''

• metro
• subway
• subway system
• tube

3. belowground, underground

under the level of the ground

Example Sentences:
'belowground storage areas'
'underground caverns'

• belowground

4. clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover, underground

conducted with or marked by hidden aims or methods

Example Sentences:
'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'

• clandestine
• cloak-and-dagger
• hole-and-corner
• hugger-mugger
• hush-hush
• secret
• surreptitious
• undercover

5. underground

in or into hiding or secret operation

Example Sentences:
'the organization was driven underground'

6. underground

beneath the surface of the earth

Example Sentences:
'water flowing underground'