subterranean

[suhb-tuh-rey-nee-uhn]
adjective Also, subterraneous.
1.
existing, situated, or operating below the surface of the earth; underground.
2.
existing or operating out of sight or secretly; hidden or secret.
noun
3.
a person or thing that is subterranean.
4.
a subterrane.

Origin:
1595–1605; < Latin subterrāne(us) (see subterrane) + -an

subterraneanly, subterraneously, adverb
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
subterranean (ˌsʌbtəˈreɪnɪən)
 
adj
1.  subterraneous, Also: subterrestrial situated, living, or operating below the surface of the earth
2.  existing or operating in concealment
 
[C17: from Latin subterrāneus, from sub- + terra earth]
 
subter'raneanly
 
adv
 
subter'raneously
 
adv

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Etymonline
Word Origin & History

subterranean
c.1600, from L. subterraneus "underground," from sub "under" + terra "earth" (see terrain).
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
But the results of the subterranean program could not be completely hidden.
But another way to get at this subterranean territory is by looking at our
  imaginative lives.
Extracting this subterranean energy is not as easy as it sounds.
Scientists who believe snakes evolved on land say they began as subterranean
  creatures.
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