Get to the bottom of something
Definition 1 of 1
Pro Tip 1/3Investigation figurative
The phrase often implies a thorough, determined search, rather than literally digging or looking underneath something. 
The detective promised to get to the bottom of the disappearance.
Pro Tip 2/3Metaphorical usage
Used metaphorically, it means uncovering layers of information or complexity to find the truth. 
Journalists need to get to the bottom of the scandal.
Pro Tip 3/3Problem-solving context
Commonly used when someone is trying to solve a problem by finding the cause. 
Our tech team is working to get to the bottom of the outage.