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• s  situated at or extending to great depth; too deep to have been sounded or plumbed
  the dark unfathomed caves of ocean"-Thomas Gray

• The President might even be involved, if only at third hand, in the still unfathomed life and times of Boston Operator Bernard Goldfine, the man that Sherman Adams vouches for.
• The biochemistry of the vast majoritymillionsof other species is an unfathomed reservoir of new and potentially more effective substances.
• For many reasons, the sea floor remains an unfathomed, and largely unfathomable, mystery.

• profound
• unplumbed
• unsounded