Synonyms for Unplumbed (same or very similar meaning)
WordNet sense 1 (situated at or extending to great depth; too deep to have been sounded or plumbed):
profound, unfathomed, unsoundedWordNet 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):
deepFrom the ODE community, based on WordNetadd/edit