Reddit user sirleechalot has build a PC in a fish tank full of mineral oil, with a Big Daddy and bubbles and gravel and everything. It's just a media PC that probably couldn't actually run Bioshock (it might be able to hand Aquaria), but it's still gorgeous.
Those of you who've been buying our paper-based website in the UK for a few years might remember our old PCG Extreme articles, where we forced various writers to go through various ordeals. We forbid Graham to sleep until he grew a beard, Tom had to charm girls and play games, and Adam Oxford had to build lots of overclocked PCs. While the liquid nitrogen case just froze off his left arm (now bionic) and didn't squeeze many frames out of the rig, Adam did find that submerging most of the PC's components in mineral oil provided a ton of cooling for such a simple and sustainable method.
There's more info on sirleechalot's original post on Reddit.