'Subterranean' is Latin for 'under the earth' — from 'sub-' + 'terra' (earth, from PIE 'to dry').
Existing, occurring, or done under the earth's surface. Figuratively: secret, hidden from view, operating out of sight.
From Latin 'subterrāneus' (underground, below the earth), from 'sub-' (under, below) + 'terra' (earth, ground, land), from PIE *ters- (to dry — dry land as opposed to water). The word entered English in the 1600s directly from Latin. Latin 'terra' is a remarkably productive root: it gave 'territory' (a tract of land under jurisdiction), 'terrain' (the lay of the land), 'terrace' (a levelled area of earth), 'terracotta' (baked earth), 'inter' (to put into the earth), 'disinter' (to dig out), 'Mediterranean' (middle of the earth/land), and 'extraterrestrial' (beyond the earth). PIE *ters- (to dry) connected dry
The word 'subway' is a half-translation of the concept contained in 'subterranean.' English used the Germanic prefix 'sub-' (actually Latin, but naturalized) and the Germanic word 'way' to create a hybrid. French went fully Latin with 'métropolitain' (shortened to 'métro'), from Greek 'mētrópolis' (mother-city), because the Paris underground served the metropolitan area. The London