penetrate vs unfathom

penetrate

verb
  • To achieve understanding of, despite some obstacle; to comprehend; to understand. 

  • To enter into; to make way into the interior of; to pierce. 

  • To move a piece past the defending pieces of one's opponent. 

  • To affect profoundly through the senses or feelings; to move deeply. 

  • To insert the penis into an opening, such as a vagina, mouth or anus. 

  • To infiltrate an enemy to gather intelligence. 

unfathom

verb
  • to come to comprehend; demystify. 

How often have the words penetrate and unfathom occurred in a corpus of books? (source: Google Ngram Viewer )