implication vs undercurrent

implication

noun
  • A possible effect or result of a decision or action. 

  • The act of implicating. 

  • The state of being implicated. 

  • An implying, or that which is implied, but not expressed; an inference, or something which may fairly be understood, though not expressed in words. 

  • The connective in propositional calculus that, when joining two predicates A and B in that order, has the meaning "if A is true, then B is true". 

  • Logical consequence. 

undercurrent

noun
  • A tendency of feeling or opinion that is concealed rather than exposed. 

  • A current of water which flows under the surface, and often in a different direction from surface currents. 

verb
  • To flow under some surface. 

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