stumble upon vs track down

stumble upon

verb
  • To discover or find something by accident. 

  • To meet somebody by chance. 

track down

verb
  • To hunt for or locate; to search for; to find. 

  • Used other than figuratively or idiomatically: see track, down. 

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