discover vs stumble upon

discover

verb
  • To find or learn something for the first time. 

  • To create by moving a piece out of another piece's line of attack. 

  • To question (a person) as part of discovery in a lawsuit. 

stumble upon

verb
  • To discover or find something by accident. 

  • To meet somebody by chance. 

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