You can't "unhide" some of the folders, and those are typically the folders that make up the Unix core of OS X. You can see them using something like Terminal.app if you know how to use the Unix commands.
I would say 30GB is a bit excessive for that, but there are other things that might be contributing. Like the package receipts for programs that come in .pkg files. Sometimes you can install programs that come in .pkg files and forget you even installed them, because they don't always show up in /Applications.
If you really want to find out where all the space is, you'll need to find some other program than Finder. It is permanently blinded to the parallel Unix directory tree. You can get it to show those directories if you explicitly tell it to go to one of them, but that's all the more you're ever going to get. Start looking for third party programs.
I'm afraid this is a very basic question (but then, the answer should be simple, too...)
My PowerBook has an 80 Gb disk, and in the bottom of Finder windows, it says that I've got around 3 Gb space left on the disk. That's of course a problem for many reasons, and probably also a reason why my beloved (old) Mac is terribly slow now.
BUT, when I, in order to clean up, turns on "Calculate all sizes" in the Finders View Options, the total number of Gb's used, according to the folder sizes, should be nothing close to 50 Gb, so there should be plenty of space left, even if an 80 Gb disk would never really be 80 Gb.
One of my guesses is "hidden files". But how do I "unhide" them?
A related thing is that I have several users on my Mac (private, work, guest, kids...) I administrate all of them, but is there an easy way to get an overview of how much disk space they use, all at once? In the actual Finder window, several of them are 24 KB, which doesn't reflect the actual disk use.
12" PowerBook (G4)
Mac OS X 10.5.8