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subterranean (adj.)[subterranean 词源字典]: c. 1600, from Latin subterraneus "underground," from sub "under" (see sub-) + terra "earth, the ground" (see terrain) + -an.[subterranean etymology, subterranean origin, 英语词源]
subterraneal: "Underground; = subterranean", Late 16th cent.; earliest use found in Robert Dallington (b. 1561), author and courtier. From classical Latin subterrāneus + -al.