Subterranean

google

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early 17th century: from Latin subterraneus (from sub- ‘below’ + terra ‘earth’) + -an.

wiktionary

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Borrowed from Latin subterrāneus.

etymonline

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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.