Webster's English Dictionary

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subterranean

sub.ter.ra.nean or sub.ter.ra.neous \.s*b-t*-'ra--ne--*n, -ny*n\ \-ne--*s, 
   -ny*s\ aj [L subterraneus, fr. sub under + terra earth - more at U]P, 
   TERRACE 1: being, lying, or operating under the surface of the earth  2: 
   existing or working in secret : HIDDEN  - sub.ter.ra.nean.ly av