"System" taking up nearly 90 GB on my mac?

I was checking my storage one day on my Macbook Air and found that "System" took up the vast majority of my Mac. I was able to move nearly all of my files off my mac and deleted several applications to no avail. I contacted Apple support, and they told me to re-index the spotlight index and such, but nothing they did helped my situation. Has anyone had the same issue and if so, how can I fix it? (I have been told to re install the OS but wish to avoid doing such because of the fear of ruining the computer.)

I'd download DaisyDisk from the Mac App Store. It will make a graphical mapping of everything stored on your drive. I have a similar problem and I ran it and there is no 89 GB sector on my disk. It's a little pricey at $10. A free option that does a similar task is GrandPerspective.