underground

under the level of the ground

• “belowground storage areas”
• “underground caverns”

conducted with or marked by hidden aims or methods

• “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”

used of independent armed resistance forces

• “guerrilla warfare”
• “partisan forces”

a secret group organized to overthrow a government or occupation force

electric underground railway

in or into hiding or secret operation

• “the organization was driven underground”

beneath the surface of the earth

• “water flowing underground”