Princeton's WordNet

1. underground, resistance(noun)

   a secret group organized to overthrow a government or occupation force

2. metro, tube, underground, subway system, subway(adj)

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

   "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"

3. belowground, underground(adj)

   under the level of the ground

   "belowground storage areas"; "underground caverns"

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

   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"

5. underground(adverb)

   in or into hiding or secret operation

   "the organization was driven underground"

6. underground(adverb)

   beneath the surface of the earth

   "water flowing underground"

Kernerman English Learner's Dictionary

1. underground(adjective)ˈʌn dərˈgraʊnd; adj., n. -ˌgraʊnd

   located under the ground

   an underground storage area; to tunnel underground

Wiktionary

1. underground(Noun)

   An underground railway.

2. underground(Noun)

   A movement or organisation of people who resist political convention.

3. underground(Noun)

   A movement or organisation of people who resist artistic convention.

4. underground(Verb)

   To route electricity distribution cables underground

5. underground(Adverb)

   Below the ground.

   The tunnel goes underground at this point.

6. underground(Adverb)

   Secretly.

7. underground(Adjective)

   Below the ground; below the surface of the Earth.

   There is an underground tunnel that takes you across the river.

8. underground(Adjective)

   Hidden, furtive, secretive.

   These criminals operate through an underground network.

9. underground(Adjective)

   Of music, art, etc, outside the mainstream.

10. Underground(ProperNoun)

    The London Underground.

Webster Dictionary

1. Underground(noun)

   the place or space beneath the surface of the ground; subterranean space

2. Underground(adj)

   being below the surface of the ground; as, an underground story or apartment

3. Underground(adj)

   done or occurring out of sight; secret

4. Underground(adverb)

   beneath the surface of the earth

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