WordNet sense 1 (situated at or extending to great depth; too deep to have been sounded or plumbed):

profound, unplumbed, unfathomed

WordNet sense 2 (having great spatial extension or penetration downward or inward from an outer surface or backward or laterally or outward from a center; sometimes used in combination):

deep

WordNet sense 3 (not made to sound):

silent

WordNet sense 4 (impossible to hear; imperceptible by the ear):

inaudible, unhearable