Synonyms and related words :
able, abysmal, abyssal, bottomless, cavernous, colossal, deep as hell, enigmatic, enormous, fathomless, gaping, immense, incalculable, incognizable, massive, mighty, monstrous, mysterious, plumbless, plunging, prodigious, puzzling, sealed, soundless, strange, tremendous, unapparent, unapprehended, unascertained, unbeknown, uncharted, unclassified, undisclosed, undiscoverable, undiscovered, undivulged, unexplained, unexplored, unexposed, unfamiliar, unfathomable, unheard, unheard-of, unidentified, uninvestigated, unknowable, unknown, unperceived, unplumbed, unrevealed, unsounded, unsuspected, untouched, vast, virgin, without bottom, yawning