stumble upon vs unearth

stumble upon

verb
  • To discover or find something by accident. 

  • To meet somebody by chance. 

unearth

verb
  • To uncover or find; to bring out from concealment 

  • To drive or draw from the earth. 

  • To dig up. 

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