Situated at or extending to great depth; too deep to have been sounded or plumbed:
— The dark unfathomed caves of ocean.

synonyms: profound, unplumbed, unsounded.

Roget 208: deep, deep seated; profound, sunk, buried; submerged etc. 310; subaqueous, submarine, subterranean, subterraneous, subterrene^†; underground.    bottomless, soundless, fathomless; unfathomed, unfathomable; abysmal; deep as a well; bathycolpian^†; benthal^†, benthopelagic^†; downreaching^†, yawning.    knee deep, ankle deep.    ... show more