subterranean

English Thesaurus

1. lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed) (adj.all)

similar	:	covert,
definition	:	secret or hidden; not openly practiced or engaged in or shown or avowed (adj.all)

2. being or operating under the surface of the earth (adj.all)

similar	:	subsurface,
definition	:	beneath the surface (adj.all)