I have a Minecraft world that won't show up in the "Singleplayer" saves list. I tried creating a world with the exact same name and replacing all the files in that folder with the files from the broken world, and through that, I've narrowed it down to one file: level.dat_old. I don't know how this level.dat_old can be so important to the game (I'd think level.dat would be more important, and _old wouldn't even be used by the game, a kind of emergency backup).
If I replace everything except that level.dat_old, the save shows up, but when I try to play it, it loads the new world I created, instead of the one I'm trying to rescue.
I've also tried copying the level.dat, and renaming it to level.dat_old, but that results in the save not showing up in the list.
I even tried pulling down the files from a backup I had, but I think they might have already been corrupted.
Is there any way I can fix this? I thought about trying something like an NBT editor, but I'm completely unfamiliar with those, so I don't know how that could possibly help.