SUBTERRANEAN

sub ‧ ter ‧ ra ‧ ne ‧ an /ˌsʌbtəˈreɪniən◂/ BrE AmE adjective [usually before noun]

[ Date: 1600-1700 ; Language: Latin ; Origin: subterraneus , from terra 'earth' ]

beneath the surface of the Earth SYN underground :

subterranean passage