SUBTERRA'NEAN SUBTERRA'NEOUS, adjective [Latin subter, under, and terra, earth.] Being or lying under the surface of the earth; situated within the earth or under ground; as subterranean springs; a subterraneous passage. [Subterraneal and Subterrany, are not in use.]
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI