Etymology
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underground (adv.)
1570s, "below the surface," from under + ground (n.). As an adjective, attested from c. 1600; figurative sense of "hidden, secret" is attested from 1630s; adjectival meaning "subculture" is from 1953, from adjectival use in reference to World War II resistance movements against German occupation, on analogy of the dominant culture and the Nazis. Noun sense of "underground railway" is from 1887 (shortened from phrase underground railway, itself attested from 1834).
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overground (adj.)
"situated above ground" (as opposed to underground), 1879, from over- + ground (n.).
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cistern (n.)

"natural or artificial receptacle for holding water or some other fluid," mid-13c., from Old French cisterne "cistern; dungeon, underground prison" (12c., Modern French citerne), from Latin cisterna "underground reservoir for water," from cista "chest, box," from Greek kistē "box, chest" (see chest). Related: Cisternal.

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subway (n.)
1825, "underground passage" (for water pipes or pedestrians, later for electrical wires), from sub- + way (n.). The sense of "underground railway in a city" is first recorded 1892, in reference to London.
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undercroft (n.)
"crypt of a church; underground vault," late 14c., from under + croft.
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tunnel (v.)
"excavate underground," 1795, from tunnel (n.). From 1570s as "furnish with a tunnel." Related: Tunneled; tunneling.
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bulb (n.)

1560s, "an onion," from French bulbe (15c.), from Latin bulbus "bulb, bulbous root, onion," from Greek bolbos "plant with round swelling on underground stem." Extended 1660s to "spherical underground part of an onion, lily, etc." Expanded by 1800 to "swelling in a glass tube" (thermometer bulb, light bulb, etc.).

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basement (n.)
"lowest story of a building, wholly or partly underground," 1730, from base (v.) + -ment.
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fovea (n.)
"depression or shallow pit in a surface," 1849, Latin, literally "small pit," related to favissae "underground reservoirs;" which is of unknown origin, perhaps from Etruscan. Related: Foveal; foveated.
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subterranean (adj.)
c. 1600, from Latin subterraneus "underground," from sub "under, beneath" (see sub-) + terra "earth, the ground" (from PIE root *ters- "to dry") + -an.
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