I have disappearing disk space

I have an iMac G5 runnng 10.5.8. It's a shared computer that's used for light work. No big files or big programs or anything. I add up the space used by system, applications, users, and library, and it's about 45G. The finder, however, says there are 128G used. For the life of me, I can't find out what's using all that space. I ran CleanMyMac, which found about 1G of waste, but that's it. What's filling up the hard drive?