This idiom or expression means:

Finding out the truth about it.

We should have been more careful. If she really wants to, she can easily get to the bottom of this situation and find out everything about our stupid plan!!

This idiom or expression means:

Reaching the lowest point possible.

Examples:
The economy is bad, but we haven’t hit rock bottom yet. Things will get even worse than this.
Some people don’t come to their senses until they hit rock bottom. Fortunately, I wasn’t one of them!!