get to the bottom of
Find the basic underlying quality or cause of something. For example, He was determined to get to the bottom of the problem. [Late 1700s] Also see at bottom.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.