I did some math research in high school where the proof boiled down to dozens of cases of ugly polynomial inequalities. I can't find the PDF now, but the final paper was something like 70 pages, and several of those were full-page polynomial expressions expanded out. The actual prose was probably 5 pages or so.

It was categorically the least elegant proof of anything I've ever seen.

I'm incredibly grateful to for the opportunity to have done the research and gotten my feet wet early on, but boy do I cringe when I look back at that paper.