When something is amazing and unbelievable, you can say that "it blows your mind":

Whenever I think about how many people live there, it blows my mind.

It blows my mind how talented Katie is at such a young age.

Sometimes people use this phrase to talk about something that's surprising in a negative way:

It blows my mind how some people can just lie right to your face.

You can also use this expression in the past tense:

A: How was the exhibit?

B: Oh, it absolutely blew my mind!