To get to the bottom of something means to discover an explanation for a mystery or to uncover the real reason for something that was unknown or undiscovered. When you get to the bottom of something you reveal the truth or solve a mystery.

1. I still haven't managed to get to the bottom of how my window got broken. It's a mystery!
2. The police stated that they were very confident about getting to the bottom of the crime.

1. After the deal fell through, I ordered a thorough review so that we could get to the bottom of what went wrong.
2. I don't know who has been passing top secret information to our competitors, but I'm determined to get to the bottom of it!