追根究底
zhuī​gēn​jiū​dǐ

to get to the bottom of

  1. 追 zhuī verbnoun
    to investigate
    to look into; to investigate; to get to the bottom of a matter; to trace
  2. 根 gēn adverbnoun
    source
    (bound form) the origin, cause, or source of a situation or problem
  3. 究 jiū nounverb
    to investigate
    (bound form) to study carefully; to probe; to research
  4. 底 dǐ adverb
    ins and outs
    The truth behind a matter; the underlying details or background information