deep
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
mineral-rich
pathless
stygian
sunless
unattempted
unbridgeable
undreamed-of
unexplainable
unmapped
unsearchable
unsolved
unsounded
untracked
untravelled
waveless