Underground; = "subterranean".
1A dwelling, region, etc., below the surface of the earth. Also (with the): the underworld.
A subterranean tunnelling machine; especially one which melts the rock in front of it and causes the molten rock to form a glassy lining to the tunnel as it develops.
Early 17th century; earliest use found in John Healey (c1585–c1616), translator. From classical Latin subterrēnus situated below the level of the earth, beneath the horizon (2nd cent. a.d. in Apuleius) from sub- + terrēnus.