1. [Noun]: electric underground railway

2. [Noun]: a secret group organized to overthrow a government or occupation force

3. [Adverb]: in or into hiding or secret operation; "the organization was driven underground"

4. [Adverb]: beneath the surface of the earth; "water flowing underground"

5. used of independent armed resistance forces; "guerrilla warfare"; "partisan forces"

6. under the level of the ground; "belowground storage areas"; "underground caverns"

7. 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 mobilizati

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