Definitions

from The American Heritage® Dictionary of the English Language, 5th Edition.

  • adjective Situated, occurring, or operating below the surface of the earth.
  • adjective Hidden or concealed; clandestine.
  • adjective Of or relating to an organization involved in secret or illegal activity.
  • adjective Of or relating to an avant-garde movement or its films, publications, and art, usually privately produced and of special appeal and often concerned with social or artistic experiment.
  • noun A clandestine, often nationalist, organization fostering or planning hostile activities against, or the overthrow of, a government in power, such as an occupying military government.
  • noun Chiefly British A subway system.
  • noun An avant-garde movement or publication.
  • adverb Below the surface of the earth.
  • adverb In secret; stealthily.
  • transitive verb To situate under the ground.

from The Century Dictionary.

  • Beneath the surface of the earth: as, to sink underground.
  • noun An underground passage.
  • noun A contraction of underground railway.
  • To place or lay underground, as an electric wire.
  • Being below the surface of the ground: as, an underground story or apartment.
  • noun That which is beneath the surface of the ground.

from the GNU version of the Collaborative International Dictionary of English.

  • adjective Being below the surface of the ground.
  • adjective colloq. Done or occurring out of sight; secret.
  • adjective See under Railroad.
  • noun The place or space beneath the surface of the ground; subterranean space.
  • noun chiefly British a subway or subway system, especially in the United Kingdom.
  • noun a secret organization opposed to the prevailing government.
  • noun a group or movement holding unorthodox views in an environment where conventional ideas dominate, as in artistic circles.
  • adverb Beneath the surface of the earth.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • adjective not comparable Below the ground; below the surface of the Earth.
  • adjective Hidden, furtive, secretive.
  • adjective Of music, art, etc, outside the mainstream.
  • adverb Below the ground.
  • adverb Secretly.
  • noun chiefly UK An underground railway.
  • noun with "the" A movement or organisation of people who resist political convention.
  • noun with "the" A movement or organisation of people who resist artistic convention.
  • verb To route electricity distribution cables underground

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • adjective conducted with or marked by hidden aims or methods
  • adverb in or into hiding or secret operation
  • adverb beneath the surface of the earth
  • adjective under the level of the ground
  • noun an electric railway operating below the surface of the ground (usually in a city)
  • noun a secret group organized to overthrow a government or occupation force

Etymologies

from Wiktionary, Creative Commons Attribution/Share-Alike License

under +‎ ground.

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Examples

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  • Fill in the blanks using the same three letters at the end as at the beginning in the same order to find a place where water flows free:

    _ _ _ ERGRO _ _ _

    June 23, 2015