how do I made screensaver?

And no, I don't mean some simple slideshow screensaver of images I have laying around. I'm talking about high-quality, animated screensavers, 2D or 3D. Example being something like the popular "Aquarium" screensaver. One that's nice to watch, perhaps sometimes unpredictable, but more interesting than a random slideshow.

I'm ASSUMING it needs to be written like any program. If so, how difficult would it be? I know a little Java and C++ and given the time i'd be willing to learn what else I need (or just steal somebody else's code) to get the job done.

thx