Etymology
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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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hypogean (adj.)

"living below the ground," 1803, from Greek hypogeios "underground," from hypo "under" (see hypo-) + "earth" (see Gaia). Opposed to epigean.

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donjon (n.)

early 14c., dongoun, "inner tower, keep, or stronghold of a castle," also "underground prison cell in a donjon of a castle;" see dungeon.

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tuber (n.)
"thick underground stem," 1660s, from Latin tuber "edible root, truffle; lump, bump, swelling," from PIE *tubh-, from root *teue- "to swell."
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U-bahn (n.)
German or Austrian subway system, 1938 (originally in reference to Berlin), from German U-bahn, short for Untergrund-bahn, literally "underground railway."
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bose (n.)
"to seek for hollows underground by ramming the ground and observing the vibrations," 1929, ultimately from Scottish word boss "hollow, empty" (1510s), earlier a noun meaning "small cask, wine flask" (late 14c.).
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sinkhole (n.)
also sink-hole, mid-15c., "sewage pit," from sink (n.) + hole (n.). As a geological phenomenon, "hole made in the earth in limestone regions by underground erosion," 1780, from sink (v.).
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pipeline (n.)

1859, "continuous conduit of pipes chiefly laid underground," from pipe (n.1) + line (n.). Figurative sense of "channel of communication" is from 1921; surfer slang meaning "hollow part of a large wave" is attested by 1963.

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Metro (n.)

Paris underground, 1904, from French abbreviation of Chemin de Fer Métropolitain "Metropolitan Railway" (see metropolitan (adj.)). French chemin de fer "railroad" is literally "iron road." Construction began in 1898.

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bomb-proof (adj.)
1702, from bomb (n.) + proof (n.). As a noun, "underground structure strong enough to resist the impact and explosive force of bombs," 1755. In the U.S. Civil War it was a contemptuous term for men not exposed to the dangers of war.
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